Darn, is it tuesday already?!
Regarding built-in chargers in mods....
I have several18650 mods (well, ok many

) with that feature. Some claim to be "pass thru" vape capable, some don't. I'd rather not be charging and using one thinking it does when it doesn't...
Charging will produce some heat, although of course nothing should get hot. But heat is a primary factor in electronics failure. I'd rather keep that heat out of the mod, away from the electronics.
And, for me, it's just too darned easy to pop fully charged cells into my mods before going out, so I don't have to worry about it at all.
But I don't think onboard chargers cause shortened life of built in cells. Lions and Lipos are only good for a certain number of charge cycles, whether onboard or off. If you only have one or two, they're going to be cycled frequently, and reach end of life sooner.