You have to realize that with luke's "training" or not, we do not see all which obi-wan had asked luke to practice. Also yoda really did not that much qualms with training luke. He was going to give it his best shot, but yoda of course had his doubts, who wouldn't?
Also the force acts on situations like "fate" luke destroyed the death star because it was his destiny. Sure it is a cop-out but it fits really well with author's prerogative, that it was the "force" (fate) that set these things in motion.
Luke was never a match for Vader; in Empire he got man-handled, in RotJ, Vader again still did not want to kill his son, and gave up.
Vader was defeated because of what Vader decided to do, with the help of luke his son.
Vader killed the emperor, Vader could of had Luke killed so many times during the OT.
I understand people wanting Luke to be an amazing "superhero" but I like that it was a little more complex, especially with the father son relationship that won the day.