Solo wise I do feel like barbarians got the short end of the stick. But they get 100 more skill in axes than knights and they have insanelly fast attack speed compaired to the knight. In a group the barbarian will far out dps the knight since you'll probably want to fight the bigger mobs which usually have very high resistances to everything but axes. As well with wizard mobs there damage will really help so they die faster.
With flails you can easily get max attack speed and can get 200 skill in without having to choose a town for it like the knight. With the whips/flails you can actually solo stuff a lot sooner than the knight like orcs, minotaurs, and ice elementals.
Like you stated though, barbarians have no sustain. They can't use the alignment skills very much or quickly run out of spirit, but still useful in pvp since your not going to be fighting very long, just untill your opponent dies, only problem is in pve.
I just see barbarians are mainly loved in groups and still have some soloability.