Generally it's judged by the types of games which have been produced. Since Unreal has had AAA games released using it, then yes I'd say so.

So then having a GC isn't an impediment to be a AAA games engine it seems.

I didn't mean to imply that it was. But it's not ideal. I believe it's optional in Unreal too, so I'm not sure if AAA games use that feature.

GC is also a very blanket term. There are many different types. From what I understand the Unreal GC gives you a lot more explicit control over how and when it runs.

Having written OpenGL applications in Haskell (not games) I can say that it has no such feature. It's GC is also vastly different as immutable functional languages generate a LOT more garbage than normal applications (measured in GB/s).

GC is not the impediment. GC as it is currently implemented in the majority (if not all) advanced type system functional languages is.

There I agree with you.

However Unity is improving their performance, you should have a look at HPC# presented at GDC 2018.