But we can say the same about Mourinho. Excluding the Porto job where he was quite lucky in the Cl and couple of referee decisions helped him win, he has gone to clubs that have way more financial resources than anyone else in the league - Chelsea, Inter, Real. I can also say it's not very impressive to take over teams that can buy anyone he wants. It's not easy to motivate players who have won it all and Moyes proved it. We were by far the best team in the league when we won the title by 11 points and got kicked from the CL because of a ridiculous referee decision, yet next season we found ourselves in 7th place. I'm not trying to compare Moyes to Guardiola, but you should take that into account. The pressure is immense when you take over that kind of job, especially when the club just won all the possible titles and the expectations from the manager are unreal.
SAF is in a league of his own - he won it all which various clubs and made them dominant - Aberdeen, St. Mirren, Utd, but you could also argue that back then there wasn't such a big gap between the teams as it is now.
Look, Guardiola doesn't have anything to prove, he can take any job he wants to because he already proved himself. And the notion that his Barcelona job was easy is ridiculous - he removed Deco, Ronaldinho (a guy most thought was untouchable, to remove him from the team was mind-blowing) and couple of other players and nobody considered him a top manager back then. To do something like that requires serious confidence. He replaced them perfectly and the team when on to win the treble in his first season. They had finished 4th, I think, in the season before he took over.
Now, you'll say he had an amazing squad, and while that's true, Xavi and Iniesta were good players, but nothing comparable to what they were under Guardiola. He elevated them massively. Then you have Busquets who he introduced in the first team, he brought Pique and Henry (I think), moved Messi to the center and they all had an amazing season. If you think that's only down to his amazing squad, then you're sorely mistaken.
Bayern is the same - he is replacing their aging players which absolute quality and he can continue to do so until he tires of the job.