He's a bit similar to our own Lewis Miley in that he was overplayed last season owing to necessity as much as anything. There was always likely to be a second-season syndrome and drop off somewhere along the line.
I think he's a fascinating player that could go either way. My feeling is that he might be better suited to playing in Italy or Spain than in England, where his lack of intensity and stamina are major weaknesses.
So for me, the main issue is what is he? A 6, 8 or a 10? For me, he's a bit of everything, a kind of hybrid, but not ideal for any role. He could develop into a 6 in his later career, but at the moment his lack of stamina and intensity are big no nos for this position. An 8 seems his ideal home, but only in a three-man midfield, similar to what Howe plays or Klopp played (though his lack of intensity would be a big issue in a Klopp team). I think a Pep midfield and style of play would suit him best. A 10? Maybe for now as I thought the intricate triangles he played with Saka and Trent (when he got on the pitch) for England last summer were fantastic to watch at times. Scholes also said that he might be best in this position for now and you have to respect his opinion! But overall, as others have said, I don't really think he's a natural 10, especially as he gets older and more mature. He can create, but he's not enough of a threat to be a ten.
I think Souness hit the nail in the head when he said he doesn't understand the position. People didn't want to hear as he was still in his honeymoon period, but he was totally correct. Mainoo often makes it hard for centre-backs to pass to him by effectively marking an opposition midfeilder. I know he is capable of playing well in tight spaces, but it can also look like he is hiding somewhat, putting further pressure on his midfield partner. Souness was a dirty bugger at times, but I think it's safe to assume that he knows his onions regarding midfield play.
Overall, I think he's worth sticking with and nurturing. As a young player, he is open to coaching and his stamina and game awareness can be improved under a good coach. In a better functioning team, he can be a real asset, but probably needs taking out of the firing line for now. That said, if you get an offer for silly money then you'd have to consider it, especially with financial fair play constraints and a new/redeveloped stadium to contend with. As I said earlier, however, it may be that the Premier League is not his ideal home.