Plan 9 sucks on lots of machines at once. Oberon is considered harmful, and sucks. Amoeba sucks, distributedly. Eunice sucks so bad Larry Wall made special mention of it. NextStep sucks, but it's pretty.
Nachos sucks, but only students know how.
Mach sucks, but nobody knows how. Spring sucks in little pieces all over the place. They all suck. Except AIX, which sucks raw eggs through a very thin straw. You may also be interested in the Operating System Hall of Shame .