The impact an engine has on solving bugs is minimal. It mostly has to do with how the game itself is structured and how new content is being implemented. If, from the start, you design it in a way that makes adding new content a chore then you're gonna have a bad time squashing bugs. And since each new update adds complexity, especially if new variables are added that interact with old ones, then you might get to a point where solving everything by yourself is gonna take longer and longer.
Now I'm not saying that's the way nosy actually designed everything. From what I have seen this game's code must be a bit more complex than what we can see, given how big updates temporarily break two of the three paths. But still, nosy has managed to fix whatever breaks (mostly) and has been pretty consistent with giving us good content.
So, in the end, it's not a easy as saying something like "It's just unity, it should be easy!".