I kind of see them the same way I see spellcheckers: they can be useful to catch some stuff, but not good enough to depend on them without human supervision. It also depends on what you're writing.
They're not very useful if you're writing fiction or literary stuff because grammar is more flexible than "if A then B." Things like emphasis and wordplay and register and idiosyncrasy and vibes are all going to modify your text.
In general reading a lot and learning about grammar and editing your own stuff is always going to be an important skill, and it will serve you better than grammarly ever will. So use it to double check you didn't overlook something obvious, but proofread the text yourself first.
All of this nuance is negated by the AI hype cycle and any piece of software that hurries to boast "AI features" that are actually terrible and depend on OpenAI not charging enough for their shitty service is unequivocally bad and should be mocked.