I signed Grealish for Forest at the same time as you, Summer 2019, as I got promoted to the Prem. I intended to use him more as a central midfielder in a DLP role, but Ben Osborn was playing well so Grealish didn't get too many minutes - that was until my AML got injured so I stuck Grealish there with an IF role. I found he can drift out of some games but on the whole is very dependable whether you play him there or back central which I occasionally do if Osborn needs a rest.
I overachieved in my first season which meant Europa League football in 2020/21, which Grealish became a main focus for in the tournament. Ours and his first group game ended with a red card - he does seem to pick up yellows quite often but I believe that to be a tactic-issue more than a player-issue.
My experience with him can be summed up in our final Europa League game of the campaign - the final. I played him AML as above, with the Inside Forward role and was tempted to bring him off around 60 minutes in as nothing was working up there (my AMR was more effective than him overall this season, especially in the league) - he then scored 2 in quick succession, leading us to glory - he ended up with 8 goals and 4 assists in the tournament, and won the player of the tournament. My league campaign was rather poor in comparison so he suffered as much as the other players.
I find that he is prone to minor injuries when fitness is low (or you have a few games in quick succession and no good rotation policy) but nothing that keeps him out for more than 1 or 2 games. I think I'll be getting a backup DLP so Grealish will be utilised solely out wide, which I appreciate is not to everyone's taste. Still no England call-up but as of June 2021, is valued at £27m (on average last season was around £18m - but Champions League Qualification and the end of season value boost is in play at the moment).
Hope this helps a bit