Believe it or not, there is a huge diehard SNL following that labels him as one of the worst SNL performers of all time. They cite him breaking character constantly -- and not in a fun way -- (typically in every sketch he's ever shared with Will Farell) and for getting kicked out of the writer's room for having horribly unfunny sketch ideas, and worse dialogue
I can't say I watch much SNL, but hell, even I can recall tons of times where Fallon was unable to pull himself together and deliver his few lines on camera