Fitting a battery and charging it via USB would be all the same(mods are designed to support that as non-ideal as it is), just plus the extra step on putting the battery into the mod..
There's no difference between plonking a battery in your TV remote and putting one in a mod, seems like a weird stipulation, but it is what it is i guess. The downside to a built-in lipo battery(if there's any discussion to be had) is having to bin the whole mod once the battery is done, you don't have that issue with mods where you install one.