Not broken.
The Eberron Campaign Setting has the Talenta Sharrash, which is a scythe type weapon. Has reach and does 1d10 (medium) damage with a 19-20/x4 crit. Exotic.
IIRC, it may have gotten nerfed to hell in errata, but I played with it as listed and found it to be fine. When I crit, the damage was stupidly high, but that's the deal w/ any x4 crit weapon.
I really don't like the high crit weapons. I prefer the reliability of a high crit range, and find too often that the high crits go to waste on someone already badly hurt anyway. Not to mention the backfire potential if you get dominated, someone casts Backbiter on you, etc... I went w/ the Sharrash for that character for style, I like scythe, but the martial version is pretty bleh (to someone who could care less about high crit weapons, again).