An entire Hive Fleet attacking in one hammer blow! Other Chapters have lost their homeworld because of a massively smaller splinter fleet invading, yet the Ultramarines along with the Ultramar Guard and the Battlefleet of the Segmentum just managed to hold off millions and millions of tyranids- at a time when there were no anti-Tyranid doctrines and they were a virtually unknown aggresor I might add.
And that's part of the problem. They're just too good at everything, and seem to always come out on top. They're like that guy at school who's good at everything. Secretly everyone hates him. Part of what makes people (and groups of people for that matter) interesting are not just they're strengths, but there faults, mistakes and weaknesses too.
When I was a young boy, I dreamed of being a baseball...