You have a point but then my whole app will need to look generic. As soon as I add my own elements I will need to start thinking "how will it look with Android button vs iOS button". It's much easier to just add my own button.
I don't think anyone will have a hard time understanding how to press a button just because it looks different than system default.
It depends. Have you ever sat down with an older person and tried to explain to them how tablets work, with them having minimal prior experience with desktop PCs? Of course, that might not be your target audience in any case...
But for the most part, it's not about the difficulty. It's just an eyesore. Kinda like that one house in the neighborhood that has bright red fences, when everyone else has green. It might even be a very pretty bright red, but...
I don't think anyone will have a hard time understanding how to press a button just because it looks different than system default.