He is very dull and uninspired, and his football is insipid, terrible. It's not the type of football I want to watch every week. He has been incredibly successful in making the English NT far less than the sum of its parts. He is terrible at making subs. And in his last club job, which was 15 years ago or something, he got relegated with Boro. Also, managing footballers playing for country is nowhere near the same as managing footballers playing for club, especially one like Manchester United. This job will eat him and shit him out in one season. Furthermore, I suspect some players might become untouchable and undroppable under him.
It's just a terrible idea all around. He simply isn't a good coach. Certainly not for top flight football. If Pep, Klopp and Ancelotti are S-tier coaches, and Tuchel, Enrique, and Arteta are A-tier, Southgate is a genuine C-tier coach. I'd stick with Ten Hag, or even have Pochettino, than have Southgate.