I hate this argument because its simply not true most of the time. Even very good players sometimes fail in top clubs where the expectations are high. For me, if you are good enough to play for the Real's and Bayern's of this world, then you are a good player. Gotze hasn't really done it for Bayern, what would you say about him then when in your logic you don't have to be a good player to play well at Bayern.
even with the case of coman aside, how is it not true, that it takes less skill to score for bayern (who totally dominate their league with no real consistent challenger)?
let's have a look at a few players that support the claim that it is vastly easier to score for bayern while being mediocre at best.
Let us start off with Ivica Olic. I would like to compare his stats while playing for Wolfsburg with when he played for Bayern. I guess we all agree that Ivica Olic is a mediocre player at best.
Stats for Bayern:
80 games, 23 goals, 13 assists
Stats for Wolfsburg:
94 games, 33 goals, 15 assists
Right now if you only look at these numbers you would say "hey, he did better for Wolfsburg". If you calculate the ratio of scorer points per game we have 0,51 scorer points per game for Wolfsburg and 0,45 scorer points per game for Bayern.
So is it easier to score for Wolfsburg than for bayern ? Wolfsburg is not a bad club at all, so is this due to the game plan of wolfsburg fitting more to a mediocre player like Olic ?
If you now look closer at not only the number of games but the actual amount of minutes played.
Minutes for Bayern: 3937 Minutes
Minutes for Wolfsburg: 6847 Minutes
If you now compare how many minutes it took for Olic to score the result tilts far towards his time at Bayern with 109 minutes for every scorer points and 143 minutes for every scorer point playing for Wolfsburg.
other similar examples of no "world beater" players that did overperform in bayern were Luca Toni (one scorer point every 107 minutes for Fiorentina and one scorer point every 87 minutes for Bayern), Roy Makaay (111 minutes for a scorer point in bayern, 138 minutes for a scorer point for Deportivo), Miroslav Klose (114 minutes per scorer point for Bayern, 150 minutes per scorer point for 1. FC Kaiserslautern, 118 minutes per scorer point for lazio), Bruno Labbadia (137 minutes per scorer point for Bayern, 181 minutes per scorer point for Bremen), Carsten Jancker (112 minutes per scorer point for bayern, 172 minutes per scorer point averaged for all of his other professional games (some even in crap leagues like Austria))
I would argue that really good players probably have a less big "Jump" in performance when they go to bayern, because they are already perform regardless of the team.
An example would be the not great but still very decent
Claudio Pizzaro (112 minutes per scorer point for Bayern, 106 minutes per scorer point for Werder Bremen) or Giovanne Elber (116 minutes per scorer point for stuttgart, 101 minutes per scorer point for bayern), Lewandowski (97 minutes per scorer point for dortmund, 91 minutes per scorer point for bayern).
This data would lead me to say that "the better the player the smaller the difference in performance when playing for bayern".
Playing for a dominating bayern side the threshold for a good strikers performance should not be a tap in here and there, but things like lewandowski showed with his 5 goals in 9 minutes against wolfsburg because they just create that many chances. If you consider that Wolfsburg finished second place last season and they still get destroyed by bayern so easily it just shows the big chasm between bayern and the rest.