Grammarly for the average user is probably an excellent experience, but for someone such as myself, while I find it to be better than most, it is still only ok. For one its dictionary is highly limited, in my Grammarly reports reports I allegedly use more unique words than more than 90% of their users. Mind you I write a lot about science and often when they highlight a word and I check it via Google it is Grammarly that is incorrect. I also find their understanding as far as "context' goes to be somewhat limited to pedantic statements. What I have found it good for is correcting typos and cerrtain grammar errors

I've used Grammarly for Firefox and have found it to be in the top 3 of the best programs I've used. It's unbelievably easy to use. I can't recommend it enough.

helpful, easy to install and to use.
a good help

Activated for all websites by default. You can exclude website one by one.
So, If you don't want Grammarly injected on every page/site and all your typing sent.
There's no option to deactivate by default and activate for only a few websites.
Probably no GRPD compliant.