Well I don't want to get too picky here, but maybe just a little bit
.....
The cells w/on-board charging
would likely have slightly less capacity than another theoretical cell with the same overall length w/o on-board charging, but in my experience they are not the same length at all. They are the same base cell, of the same length, with same capacity rating, to which the charge module is added. So in my experience, the penalty is not reduced capacity - the penalty is increased length. As I indicated, that length can be a big deal for certain lights - a good example in the 14500 format being the EacTac D25A (which doesn't have a mm to spare). I don't have calipers handy, but it just happens that I ordered two pairs of 18650s yesterday which are the same cell, one pair with and one without the charge function (both being 'protected' cells). The former are listed as 1 mm longer than the latter, and yes, the ones with the charge module are a whopping $0.98 more expensive than the standard cells.
<<Also, with a separate charger, you can be charging one cell while using another cell in the light.>>
This is the part I'm not getting though. One can plug in a separate charger (like the double-magnet type you mention) into a USB source and connect the magnets to a cell, or one can plug a USB cable into that USB source and connect the other end directly to a cell w/ on-board charging (Micro USB in the case of mine). In either case, I can (and do) have another charged cell in the light and am using it. I think this comment may have applied to
flashlights w/ "internal" charging, rather than
cells with internal charging - but that's another subject. The OP
might be looking at those too.
Yeah, I'm sure the charge module that's added to the cell can also fail, and while I wouldn't know the MTBF, I imagine enough physical abuse might cause a failure also. I haven't had any fail and don't know the failure modes, but I guess it's possible it could render the cell useless. As long as it
'fails safe', it wouldn't make my list of things to worry about
Dig that tabby cat, man!