Is it possible to subsititute nutella for peanut butter?
Nutella is over half sugar by weight; peanut butter might be more like 5-10% depending on brand/style. That already sounds like enough that it won't make a good one-for-one substitution in most things, especially anything that's not already pretty sweet.

It probably also means that it'll have a slightly different resulting texture; nutella is already a bit less thick (probably again affected by the sugar), and I believe this is pretty obvious when heated, though it's been a while since I tried.