fair enough place to start, I think your next move would be to try one yourself… My experience is very positive.
Ive got half a dozen USB rechargeable cells, couple Fenix brand, couple Olight brand, couple Lumintop brand. Been using them exclusively for the past 3 months. They work great!
I check them with a voltmeter after they are full, and they have always been up to 4.1x, never 4.2… so I trust they dont overcharge. As reported in other posts above, the charge rate is less than .5c, so all good there too… the batteries do not get hot while charging either. I also like that the Overdischarge protection works as intended, cutoff is at 2.7v
I dont recommend the Fenix, not because they dont work, only because they are longer. Here is the Voltage on a freshly charged Fenix USB 16340:

I like the Olight brand very much, they have also been 100% reliable. In this next photo you can see an Olight USB 16340 while charging, plus one of the Lumintop cells in a PicoPull XL capsule, that I also really like. There is also a CR123 in an Oveready 18350 capsule, and the Olight Universal Magnetic charger in the pic:

The Lumintop Brand that were included inside my Utorch S1 Mini have also been consistent at not overcharging. With one glitch, the Red charge indicator LED on one of the Lumintop batteries, went through part of a charge cycle without turning on. But the inline USB charge indicator I use, showed it was charging. About 5 minutes later, the Red charge indicator light came on.
The Green Charged LED worked. I have continued to use the battery without issue, and on the next recharge the charge indicator worked fine…

At no point did the battery get hot, and even while the charge indicator light was on break, I could see on my charge current meter that the charge rate was proceeding fine, not too high, not too low. As Goldilocks said, Just right.
Lastly I have been using the charge circuit in my PT-16 and PT-18, which is USB and looks very similar to the USB batteries, for over a year, without a single issue.
afaict, a charge circuit disc is reliable at not charging too fast, and reliable at stopping the charge below 4.2v.
I have also watched the voltage and current meter that I have inline with my USB battery charge cable, and Im happy to report that once the cell is full, the charge rate drops to complete Zero. I like that. I did not want to see a constant ongoing charge current… So again, very happy with the design of the charging discs. I still dont leave LiIon on charge unattended, and I charge on a metal surface, in case they were to “vent with fire”, but I have had no hint of any thermal runaway.
I recommend the Olight brand USB charging Protected 16340 without any reservations.
Shortly I will receive a Klarus Mi1c, that also comes with their own labeled version of the USB battery. If there is any problem, I will report back. Until then, unless you can find any other issues reported, I would say the USB chargeable 16340 are completely safe to use, and I give 2 thumbs up.
imho it is safe to say that there have been zero overcharging issues with the USB charging circuits. I like the USB batteries very much and recommend them highly. Super convenient to be able to charge from an Android phone cable, no need to carry a separate charger. otoh, the USB 16340 cells also work fine on a standalone charger. Zero issues there either.
Here is a little kit with spare batt and charge cable:

Lastly I can also charge the USB cells using the Olight Universal Magnetic charger… though imo it starts out a bit high, slightly over 1C:

it tapers off reasonably well as the charge level rises. The reason I bought the Olight Universal Charger is because it is versatile, and will charge not only LiIon, but also Eneloop. Whats not to like?
Here is the Olight Universal Magnetic charger, showing zero current when the charge is complete, as confirmed by the green light.

Here are some of the various charging options I have mentioned in this post:

and a Camping Kit with multiple choices for charging, battery carriers, and chemistries
