Grammarly is definitely safe to use.
There is no way that you can get a virus FROM Grammarly ( unless it is a pirated version, fake version, trojan computer virus, etc)

No, it is not safe as it is gaining full access to all of your sensitive data in your writing.

In my experience, it is very safe. Since I use a Mac, all of my use of Grammarly seems to take place on their server and site rather than on my laptop.