I have a theory about football managers and how they get to be successful / unsuccessful:
Most managers are much of a muchness, ex-pros with a good knowledge of football having been immersed in the game from a young age.
Then at the top, there's a small band of genuinely good managers.
And at the bottom, a small band of genuinely shit managers.
With a fair wind (say a couple of signings come off at the same time as some great youth comes through from the academy...),
any one of the middling managers can have a fantastic season.
But if the wind blows in the opposite direction (terrible injury list, star players poached etc.) the very same manager looks a chump.
A good example of this lottery would be Danny Wilson.
At Barnsley, he makes 4 or 5 signings that all turn out great, result: promotion.
At Wednesday, awful signings and fall outs with top players, result: relegation.
At United, has a great squad and budget for the division, but then loses leading striker in bizarre circumstances...
Also, never forget to take into account levels of expectation around specific clubs and how that affects perception of success / failure.
If a team's spent several years struggling against relegation, then even getting near the playoffs feels great...
However, a big club performing well below it's historical average can be akin to a hornets nest.
So, in my opinion, getting a "good" manager is a question of luck divided by expectation...