Charging slow-charging North American devices in Europe?

Every voltage adapter I've looked at warns against its use for electronics, for extended durations, or unsupervised.  This is just the opposite of the charging requirement for my electric toothbrush.  Googling indicates that voltage adaption is fraught with risks.

Is that really the situation, or is there a safe, reliable solution?

To some extent, you must recognize that legal warnings often say things like "We aren't responsible if something happens when you use our product".  But to be honest, simply breathing carries it's own risks.  Face it.  Living is dangerous.

On the other hand, a simple google search will provide you with infinite returns of homes, etc. burned down by electronics.  I, personally, have a friend whose house burned to the ground last year.  The Fire Marshall says the fire was caused by him leaving his battery charger for his screw gun plugged in overnight.

Bottom line is… electronics are dangerous.  The "safest" thing you can do is follow manufacturers instructions and only use the manufacturers devices to charge with.  Beyond that… you roll your dice and take your chances making as informed decisions as you can.

I think you're answer is a balanced view of the uncertainties.  In this case, the voltage adapters' warnings are much more emphatic than the typic CYA disclaimers typically found on other products, so I will avoid using the generic adapters.  Fortunately, the maker of the electric toothbrush has responded and cited its own voltage adapter.  I am currently inquiring about how to get it within my home country.  If I can't, then it might be a sign to join the current decade and get a new electric toothbrush and adapter, assuming that the newer ones are able to plug into 120v or 220V.