>Those choices can constrain your execution and to some extent, your imagination.

In theory, yes. In practice 99.9% of the games developers want to make are feasible with an off the shelf engine.

While true, people overestimate how limiting it actually is. It's like wanting to make your own kernel for your application instead of using an off the shelf one like Linux. Sure you might be able to eek out some extra performance but there are so many upsides in reusing Linux.