I've found that making a custom engine is more amenable to teamwork, not less.

I can build a simpler level editor that I can give to a totally nontechnical person (a.k.a. my spouse), and they can go to town making levels.

They don't need to know anything about Unity/Unreal. They just need to know about my game.

My game already includes levels my friends have designed using tools I made for them.