Surrendering with Amumu usually isn't a good idea.
I've gotten a lot of teams surrendering early when I play Amumu (or attempting to). When they fail, we usually win. When they don't...well yeah. This goes more so for if the enemy has a fed assassin early.
Now I realize, getting your face stomped in early sucks, but it's very hard to siege a team with a tryhard Amumu, even when fed if you're all still putting 100% effort in. If you hold until full builds, it's very hard to win against him, particularly if you had early game champions and they did hold off the sieges.
In addition, for me anyway, if I picked Amumu, it was probably because the rest of the team picked a poor team composition, and Amumu fills a lot of gaps in poor team compositions. But doesn't mean you can't win late.
Obviously some just want out, but I hope more people will take this into consideration.
Thanks.