@Killer Angel and
@Lord Agragax of Lunaxoatl, I know I'm in the minority, but I never minded Ewoks. Naturally, I think most of this goes back to seeing the OT movies in my youth. Watching Return as an adult, I realize that Ewoks winning against stormtroopers does stretch credulity. Obviously, I think it plays to one of the major themes of Star Wars, which is the natural world, the spirit and/or the underdog being greater than machines (or "the machine"), oppression and tyranny. But, it is played a bit heavy-handed to pit simple Ewoks against the Empire.
To over analyze it, there are some things to consider. I never got the vibe that the Imperial presence physically ON Endor was "overwhelming". The Ewoks were never even factored in as a threat by the Empire, thus they were overlooked. There were a lot of Ewoks. The Ewok surprise attack threw the battle into a chaos that allowed the Rebel strike team to start the battle in earnest. In some ways, the Ewoks were more of a distraction force to allow the fewer, but superior (for the sake of the story) Rebels to fight back. I don't know when the Ewoks had time to set up all of those traps, but you could assume that they had been preparing to fight back for some time. And... apparently, stormtroopers are generic fighters at best and their armor is primarily decorative.
Gungans on the other hand I don't like, and I'm sure it IS because I'm not a fan of JarJar. Plus, it's also likely due to seeing the PT as an adult, with my own opinions about what the Star Wars universe should be like. However, Gungans just come across as slapstick for slapstick's sake, and they have an odd vibe of being primitive yet with advanced technology. Conveniently, that technology is perfect for killing droids. Regardless, once again, it is part of the Star Wars theme of the natural world overcoming the machine world.