The important thing for safety is to charge
batteries in a safe place that won't burn and to not charge overnight, regardless of whether you use internal or external charging. I've read of charger fires and of
mod fires while charging. The quality of the design is important so one model may not be just as safe as another. You could have a
mod autofire while charging, you could damage your battery wraps taking them in and out to charge externally. No one has the definitive answer on which is safer imho