Manicini is far from being a bad coach and is a very good one compared to the ones now leading most top clubs. He is definitely better than Rodgers, Wenger, later Benitez, Pellegrini, Enrique, Mazzari, Inzaghi, and Allegri. He had a free ride in Serie A with Inter that is certain but he also did build a very good squad and he always had a very good eye for talent. The purchases of Maicon (who was a nobody) and Samuel + Cambiasso (who where flops) were brilliant for Inter. Aguero, Silva etc... are brilliant for City. He also built a very good Mancity squad and his players have always benefited the teams he coached both in the short and the long term.
He is no Mourinho, Klopp, Simeone or Conte in the sense that he never over-achieves but there is still some very good aspects to his coaching that justify his hiring. The situation at Inter is a mess though so I doubt he'll be able to do anything great with the team but I think he'll do much better than Mazzari. He'll lay the foundations for a good team relative to the budget constraints that he will have. They'll do better in the league.
His main weakness? Obviously the CL. He is actually one of the worst out there. All things considered he is Inter's best chance of re-joining the top-3 and his presence should make Inter more interesting.