I think the PL is a different ball game now compared to when Pompey were in it. At Leicester and Sheffield Wednesday the clubs hardly prospered under him either.
Quite the contrary.
He picked Leicester up when they were in dire financial trouble and although they suffered a relegation in his first season they responded emphatically the following season under Nigel Pearson to earn promotion. The following season they lost to Cardiff in a penalty shoot-out in the play-offs. He's then sold them to this rich consortium they have now who have since taken them to the Premier League.
The circumstances at Sheffield Wednesday were similar, too. A club that were ravaged by debts and were on the brink of liquidation. Milan brought the club, paid off a lot of their debts and settled others and eventually set aside turning around a club which were in a relegation battle in league one. He sacked Irvine, appointed Megson and they stayed up comfortably in mid-table. The following season they were promoted with one of the highest points tallies seen in league one, over-taking their rivals. The following two seasons have seen them consolidate themselves in the championship on a low wage budget before selling the club to a rich Azerbaijani businessman with a proven track record of running clubs appropriately.
He's not brilliant but he has done a superb job in turning around clubs which have been hampered financially.