There was a period of several years in which Unity was *the* best choice for a 3D engine, if you were a small studio. (And one of the top contenders for 2D, as well.) A generation of game designers was growing up who started with Unity and expected to stay there for a while.
I figure (although I haven't seen a direct acknowledgement) that Unreal's license change, opening up indie and low-budget licensing models, was sparked by the fear that Unity was becoming a universal assumption across the game industry.
As to whether Unity was uglier, slower, and buggier than Plasma -- I'm afraid I have great respect for Plasma's bug-displaying abilities. :(
Andrew Plotkin -- Seltani founding member