The main criteria to rate a coach include winning trophies, leading a club out of a lower division and in some cases, the revenue he generates for the club. Mikel Arteta is not growing young players that will bring huge financial returns like Arsene Wenger and Diego Simeone of Atletico Madrid. The club had kept investing in players only for the team to end up in the second position for several seasons. Yeah, he is a good coach, but he has not entered the books of successful coaches in Europe.
At least he is not a bad coach. He is not a successful coach, but he has good abilities in strategy and maximising players. The issue of obtaining trophies depends on the level of competition in the league. We know how competitive the Premier League is; being consistently at the top of the table for several seasons is clearly not easy. Many teams end up experiencing a decline in performance, even changing coaches, but it does not yield proportional results.