He isn't but he isn't a match for Caedus or Sidious, and Vitiate can at least keep him pressured long enough for The Emperor to kill Yoda with drain or outduel him and kill him etc. Also Revan or Yoda can't kill him either way, he can use essence transfer and possess either one of them.
He can't do that to Luke though.
DE Sidious could win...but it's unlikely. I'd give him something like 3/10 vs. GM Luke.
However, Sidious could and would draw that fight out for more then long enough for Yoda vs. Caedus to be resolved, as Sidious will obviously be hoping for Caedus to win and help. Luke, meanwhile, definitely does not want to fight his archnemesis and his nephew at the same time.
Still, if Jaina Solo Fel can take Caedus...it's a petty long argument to say Yoda can't. Jaina's feats really don't match up well to Yoda's.