I have just re-watched Return of the Jedi, paying specific attention to the scene with the Ewoks. And in fact, I think it's quit believable!
To start, you don't see any stormtrooper get KILLED by those falling rocks or arrows. The good and PROVEN tactics with the rope spanned across two trees (same happened in Tomorrow Never Dies, a Jams Bond movie, which makes it a 'modern' tactic) did kill some speeder bikes troopers.
Other (logical) tactics did so as well. The logs rolling of the hill, destroying an AT-ST? We saw in TESB that the legs of these walkers is their weak point. It makes sense.
Then the spreading of Forces. Han Solo and Leia are left alone at the bunker? I think not. An aimed shot disables R2-D2, slowing their proces. When Leia and Han seem to get in, another shot wounds Leia. So the Stormtrooperd WERE paying attention. Plus,there are still 3-4 groups of Elite Soldier inside, since the Imperial commander orders them out at the end. So the 2 or 3 Rebels outside aren't a real threat to the base itself. That's why they go after the Ewoks.
To conclude, the only reason they actually win this...is Chewbacca! He takes an AT-ST. And THAT AT-ST is the ONLY 'good guy' seen killing Stormtroopers, in fact killing a whole lot of them. This makes sens as well, since the Elite stormtrooper group there wouldn't be armed against tank and vehicles alike.
So...any thoughts? Or Ewok bashing?