[sloopy543]

In my experience, it was easy... until I got to the part of my game that makes it unique. Then it went from easy to very very hard.

That's the thing about engines. They'll get you up and running quickly for basic stuff, but you'll be tearing out your hair when you need to do anything non-basic.

[megameter]

The way I put it is, "the engine isn't really done until the game is." So many polish features in games lead to new categories of assets, which in turn need new methods of rendering.

This is one of the multitude of reasons that so many older game devs reach for the C++ compiler; they don't know what the game will need, but they do know they can make the necessary modifications to ship if they approach it bottom-up.