It's worse than a Great Sword that does 2d6+2d6 on a crit but 2d6 vs 1d12 all the rest of the time. That is unless you crit more than 1 hit in 4 & the extra damage matters.
Actually, doesn't a greatsword just do 3d6 on a crit? (How to Play, p. 16)
It's actually a problem in the rules:
Longsword (one-handed) crits at 2d8 (avg. 9)
Longsword (two-handed) crits at 2d10 (avg. 11)
Greatsword (two-handed) crits at 3d6 (avg. 10.5)
and, for good measure:
Battleaxe and morningstar and war pick (one-handed) crits at 1d8+1d12 (avg. 11)
Glaive (two-handed) crits at 1d10+2d6 (avg. 12.5)
Great axe (two-handed) crits at 1d12+2d8 (avg. 15.5)
Warhammer (as longsword)