What I dislike most is the weakness of the castle's magic.
An upgraded Mage, even with a not-so-strong army, will easily defeat the Swamp creatures.
Since I like large maps and long scenarios, I don't play as the Swamp, but I admit that in online games, on smaller maps, they can be very good.
P.S. I've been playing Heroes for 7 years (although only against the computer :-), maybe now I'll finally sign up for some tournament). And only ONCE did the Swamp creatures cause me major problems, but in the end, I defeated them :-).