You can not make general statements like this… well you can but its just wrong.
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The only time I charge a cell inside a flashlight is when there is no other option, like with most keychain flashlights.
Otherwise, I use my Vapcell S4 Plus to charge any cells. 
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In the best case, when the operator knows about liIon cells, using a good external charger will mean better safety and longer cycle life for cells. Simply reducing the termination voltage slightly and using lower charge rates are well documented as extending cell life. There is no question about this. Being able to monitor cell IR and resting voltage will tell you when a cell is aging and may not be safe to use in demanding situations. Meaning better safety.
Chip based USB BMS do not allow for any of this. Indeed most user will never know if their cells are being over charged. I have three different lights with USB charging that charge to over 4.2 volts. One to 4.26 volts. I wouldn’t have known had I not checked.
Maybe it is not accurate to make unequivocal statements about anything being universal. But For the best case with internal or external chargers, there is clearly potential for monitored external charging do do a better job. While enhancing safety and long term cell health.
Bottom line is that I don’t trust the cheap USB charging on flashlights I will not use them at all except in limited circumstances. And those only when I have done the measurements to verify that light is not cooking my cells.
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This all requires an operator who actually cares enough to educate themselves and then make a decision about what they think is the best compromise between battery capacity, charge time, and longevity.
But this is:
- a compromise,
- requires knowledge
- you could also argue that, with some chargers, the user can even increase the charge voltage to 4.3 V.
IC-based charge controllers also allow the charge endpoint to be set, but it has to be configured by the manufacturer rather than the end user.
Again, an end user with the right charger could make a more conservative choice, but they could also do something stupid. So having the maximum charge voltage set by the manufacturer is not automatically more or less safe, nor is it automatically better or worse for battery longevity.
This also only applies to some complicated and expensive chargers. Many simple chargers use the same charge controller ICs as some flashlights. Some are even worse (see video).
And in the same way, you don’t know with an external charger unless you check it.
We are taking about for well known “good” batteries and chargers, and not for the very “cheap” ones… !
Im charging multiple batteries for more than 15-20 years and i never had any problems, with both of my chargers XTAR VP 2 and Vapcell S4 Plus V3.0 !
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And this is why
Sure In SOME cases you can an external charger can be “better” but this is absolutely not the same as “It’s always better to charge the batteries on an external battery charger,”
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Well it is for me… So that is what I do.
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That reminds me of a joke…
“All generalizations are false.” 
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Is country miles away better to charge the batteries on an external battery charger, and to have some extra batteries for every flashlight so you can rotate them, and use the on board charging port of the flashlight only when you don’t have another option !
always
adverb
without exception; on every occasion; every time
It’s without exception; on every occasion; every time better to charge the batteries on an external battery charger?
Clearly not. since i provided an example of the opposite. so the statement is false.
can you provide some data to back this up or reasoning on an engineering level?
Because this sound like a very Hand-waving explanatio.
Sure some flashlight might have a non ideal charge controller… ( but so are some external chargers)
And today i would argue most quality flashlights with a USB port charge the internal battery just fine. no nead to worry or even fearmonger
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Don’t play with the words !
The “unwritten rule” is always better to change the batteries in an external battery charger, when it is possible of course !
This is the meaning of the words you say 
And it is not" always better to change the batteries in an external battery charger. "
it might be "in most cases better to change the batteries in an external battery charger. "
And even for this you have no evidence to back up this claim.
it surely is not without exception. since i presented exceptions
Are you in vacation in a little tropical beach… !?
There’s no reason a quality flashlight manufacturer couldn’t achieve the same, but sales would be terrible as (nearly) everyone dislikes proprietary cells/batteries.
Do you need proprietary cells? Why can’t we just use generic 18650/21700 but then have a BMS control the system itself? I’m asking since this power tools are just a lot of 18650s with just better BMS?
I can’t see your other posts, did you post on here or Reddit? There are plenty of lights that offer built-in USB C charging, your appetite to risk will determine which you eventually choose.
My other post is on here Looking for a 1x 18650/21700 EDC Light for Travel with USB-C to C Charging? - #25 by Kitty. The TC15 V3 is what I settled on but lost it in a bag so I got the FFL X4 and using it since.
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I think this has already been answered and I’m not sure if you are being pedantic here or generally want to understand. I’ll assume the latter.
Integrated charging in lights varies from chip to chip, as has been noted elsewhere. Batteries typically suffer reduced longevity if they are discharged too far or too fast or charged too far or too fast. Cutting off charging at 4.1v tends to be better for battery longevity. So does charging more slowly.
Some integrated chargers are low current but others can be 2amps or more. For many batteries, that’s perfectly fine, whereas some may benefit from slower charging, particularly smaller and lower CDR ones.
TBH it’s too much work for me to pay attention to which flashlights do a good job charging and which ones don’t, so I’m more likely to use external charger just because it takes the guessing game away. Of course, there are exceptions to this where I either don’t care or just prefer the convenience of integrated charging, The theoretical loss in battery durability isn’t that meaningful. I have more batteries than I need and newer better ones appear on the market annually.
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I’m being pedantic with overgeneralization.
In other ways, I have a pretty good understanding of battery technology and electrical engineering. So I know that lowering the charge cutoff voltage, as well as reducing the charge current, can increase battery longevity
within reasonable limits. It’s always a compromise.
However, there are many flashlights that charge the battery at only about 500 mA, while some external chargers can charge at up to 4 A or even more.
I also have some flashlights that reliably charge to 4.1 V, while I have external chargers that would charge to 4.25 V if the battery were left in them for too long. So there are always counterexamples.
It’s like saying that a fancy coffee machine always makes better coffee.
Well, if you know what you’re doing, you might be able to make better coffee with a professional espresso machine and freshly roasted, freshly ground coffee. But that doesn’t mean there are no good automatic coffee machines. It depends on the design and how it is used.
There is Plenty Flashlights that charge the battery just fine and save.
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Some don’t. Personally I prefer to be making the decisions and verifying the results myself rather than leaving it to chance.
I really will not concern myself about your preferences. You do, you.
From testing, the FC11, TS22, TC15 v3, X4 as well as the LT1, LT1S all seem to charge fine for me internally. I’m not sure if those are all tried and true for you though but curious if charging issues are due to brands or moreso age (when they are released) or something else?
One call out for me is that people seem to forget that opening/closing does cause wear on that part so it’s not free in that sense.
I’m calling it out since I’ve had a MacBook where one of the USB-C ports will not take charge anymore since I plug in to charge and plug out when not using it, analogous to your comment. My Lenovo laptop which I leave more plugged in lasts notably longer there but there is failure that can happen when constantly replugging in devices over and over again. Both devices have the Thunderbolt, charging, CPU and memory on the same board so it is an expensive repair. I asked the repair man and he said that it happens because the port does too much but also wears down so it’s inevitable.
Not sure for those that do swap out cells often if there are issues with the thread or cell due to it. Arguably replacing a cell is a lot cheaper than a motherboard but curious how frequent it may be for folks here.
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How does one measure how good charging is on a flashlight? There’s no screen on it or do people just plug in something to complete the circuit and move the voltage to see what the output is?