Plenty of "serious" games are made in the engine -- Genshin Impact, Hearthstone, Marvel Snap, Among Us, Pokemon GO, Beat Saber... is there a reason those games are worse than something made internally?
On a lot of those, the engine is more of a bare-bones renderer: only one Scene is really used, all entity instantiation and level loading is done by a user-land script. Some physics/collision was very custom too. Editors were also almost always custom, on larger games you write custom tooling, mid-sized ones preferred packages from the asset store.
Perhaps things are different now. Character movement was 100% offloaded to packages, but now there are more capable systems (I haven't kept up). But I still see a lot of chatter about how to do this kind of thing in forums, etc. Anyway, that's what I remember seeing, so: IME and YMMV, of course.
hehe, no. Things are pretty much the same. Source: I'm working with Unity daily.