Sure there are tradeoffs but they still are concrete decisions. Do I care about X or Y more.
Making a game fun is a bit of a black art. A little more juice (screen shake/SFX/etc) can make the exact same experience otherwise a lot more fun. An interesting recent example of this for me is playing Witchfire, a 1st person single player extraction shooter that just came to steam in early access. One of the bolt action guns once you level it up makes a bell sound with every headshot that makes it feel incredibly epic, along with the benefits headshots give for that gun.
There's pretty much no rules when it comes to making a game. Versus with an engine, you at least have some requirements and guidelines. It's gotta render stuff, do physics, etc. With a game you could literally do anything, and 99.99% of all stuff you can do probably sucks and isn't fun. So you have to really be willing to throw ideas away.
This doesn't sound right to me. Surely making an engine has thousands of different choices to make and thousands of rabbit holes to get lost in.