Sidious is most powerful sith and luke:
“Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history.”(The New Essential Chronology, page 84 )
I find that a bit unfair actually, they fought on Sidious' homefield sort of speak, it was crawling with Palpatine's loyal soldiers around.
Had they been fighting in a Rebel controlled base I think Yoda would have re-engagded after he fell off the "platform". And Sidious would probably been trying to get out of there. Yoda only had so much time until Palpatine's minions would get there and interfere.
I'm not claiming Yoda was stronger or would have beeten him on his "homecourt", but it was a draw and Yoda had to leave before Palpatines soldiers arrived, and they did shortly after the fight, he probably sensed it.
Also Luke was a wimp who got a crashcourse from Yoda, Since Lucas haven't written any of the EU stuff EU doesn't count
Like Lucas said in the 60 minutes interview when they asked if he would make episode 7-9. "There is no episode 7, and never will be". Can't remember word for word, but he made it quite clear that it had never ever been intended to be more that 6 episodes, it was 6 episodes from the beginning, he just had to do 4-6 first for technological reasons.
So Star Wars is episodes 1-6, all else is just "unofficial" or "Hey, this could make me more money, so sure I'll approve a licence".