[aragilar]

Uh, pdftex, xetex and luatex should support everything of the original tex engine, but each has extended tex, so some things will work on pdftex (which I think is what you're thinking of as plain latex), others on xetex and then others on luatex (or pairs of engines, I know microtype works on pdftex and luatex, but not xetex). I don't think this is an tex specific problem, more a natural result when there's multiple implementations.

I suspect as typst only has a single implementation (I believe), it won't have the problem of different engines ;)