I'm not entirely sure I can eloquently put it into words, but I cannot see how Grammarly could possibly be trustable or even how trust should play a role in the decision to use it.

I mean it's a service that basically tracks every word you write, and does so for what I consider a very small benefit. Even if you can trust Grammarly to be completely ethical today and in the future, no service can claim that it's completely secure from hacking.

All OSes have what is essentially the same service as Grammarly, without having to stream every word you type to a third-party service.