In general? He is good at carrying the ball forward, he is good at pinging balls forward into the attackers, he is good at receiving the ball under pressure and beating players on the half turn, he is good (when given the opportunity) at long range shooting and at bagging goals in general, he is good physically and in the air.
He is also homegrown, which folk may laugh off as romanticism etc, but on a very practical note the club has homegrown quota it needs to hit, so it makes sense to keep someone like McTominay in the squad over a comparatively similar (though in my view more limited) player in Fred, who doesnt do anything to meeting that quota.
McTominay's main weakness is his defensive positioning and reading of the game. He cannot dictate tempo or play those big switches of play reliably, and he sometimes seems to hide from the ball (although I dont think this is actually the case, its more about having that synergy and understanding with teammates of positioning - the positions McT takes up are not those that you would want your DM in). In short, his skillset has very little overlap with what you would want from a DM. Put a Carrick next to McTominay and we would see the best out of him - which for me is a very good player.