What goes wrong is that Lucas plays to their appearance and not against it.

So what should be a tense battle becomes a series of slapstick routines where cute bears hit clowns in stormtrooper outfits with sticks and rocks.

Instead we should see a legion of stormtroopers (on top of those already in the garrison protecting the dish) taken by surprise by pre-planned piece of guerilla warfare by a stone-age civilisation who had already engaged the troops in the garrison on a smaller scale before the Rebels even got there.

How else would all those traps get there if they had so much short notice?

The Ewoks must have intended on attacking the forrest around the bunker long before the Rebels landed.

Their village should be decorated with the corpses of their enemy.

That way their appearance (teddy bears) becomes a dramatic surprise instead of a joke which undermines the menace the Imperial forces have represented for the last two and half films.

With the Rebels added to that, then it becomes a plausible sequence.