HKJ has done testing with examples of the variance in cell quality and in charging quality - http://www.lygte-info.dk/
I absolutely wouldn't want on board charging unless it is absolutely necessary for the intended function of the light, implemented properly, tested to verify that it is operating correctly, and from a manufacturer with a reputation and history of good quality control. For instance the BLF LT1 would not be as functional without on board charging, but it was done well. There are too many extra opportunities for problems to happen and too many manufacturers that cut corners with QC for me to say I am for onboard charging in general. I just carry an extra battery or 2.
I am for removable batteries in cell phones and still daily use my old S5 with a removable battery where the charging port has failed, and I remove the battery to swap it out when it gets low. It gets annoying, but I am glad I have that option, and eventually I'll get around to replacing the charging port on the phone. One huge caveat to cell phones and laptops using onboard USB charging are that unlike other USB charging devices, they use computer controlled and monitored charging with much stricter QC than any flashlight that I have heard of. I don't entirely trust the quality of my external phone battery charger, but I have verified that it slightly undercharges the cell and charges very slowly at a low current, so the cell never gets warm.
Just my preference, but I won't gift a Li-Ion light(and a tested, reliable external charger) to anyone who can't be bothered to treat the cells with the care they warrant and who hasn't been informed of the difference between Li-ion and primary AA batteries etc, and safety in handling them. I don't want to contribute to someone being injured or killed from a lack of understanding or from a manufacturer who puts profits above quality control and safety. A few years ago in a city near me, there was a little girl who was killed and her siblings severely injured in a fire caused by a "hoverboard" and the dangerous implementation of Li-Ion power where catching fire was so common that they were banned from being sold for a time and removed from store shelves if I remember correctly. Many clear cases of manufacturers cutting corners on safe implementation of Li-Ion and in quality control.
There are also reports, though rare, on BLF of people who have had cells charging externally, and they exploded, but if I remember correctly at least one case was a cheap 14500 ****fire cell. ***Edit I found it: https://budgetlightforum.com/t/-/11570
For anyone interested, another good source for everything battery related including info on variances in chemistries and on the charging of Li-Ion cells is https://batteryuniversity.com/index.php/learn/article/charging_lithium_ion_batteries