how much can an iOS actually do on MacOS
Doesn't matter, as this is the wrong question. The question should be: Given that you have a real computer with a real monitor, have real processing power, significant memory, and expansive screen real estate galore, why on <insert your favorite deity here>'s green earth would any sane person prefer to use an "app" designed with a cellphone's limitations instead of a real application that can fully utilize everything that you have?
The number of advantageous use-cases for íOS on MacOS are very few, and are mostly just games.
Just because you can do something doesn't mean you should.