> All you should need to know about a charger is the max wattage.

Right. It should be that easy, but in practice not everyone is going to implement the spec correctly. If you call Apple support when someone else's power adapter doesn't work I guarantee they will tell you that only official Apple power bricks are "supported".

At the moment, in practice, I've mostly seen USB-C chargers showing up with newer tablets, not laptops. So if you need to bum a charge from someone you're probably going to get a charger that's too small.