Everyone hates the Ewoks. This seems to be the universal consensus among legions of Star Wars fans, and one of the reasons why Return of the Jedi was thought to be the weakest of the three films. Then, of course, Lucas made three more films, thereby elevating ROTJ from the bottom of the pile and dropping the Ewoks from "most hated" status. I expect they're at least a distant third now.
I have to admit, I'm not everyone, and I think some of that is rooted in my academic background. One of the chief complaints I've heard about the Ewoks is how they managed to overcome the obviously technologically superior Imperial Forces. (Speaking of which, why is it "Imperial" with an "I" when it's "Empire" with an "E"? Have to find that out one of these days.) This, more than any other argument I've heard, seems to be the main source of ire. I suspect some of it may also be that it was Ewoks, not Wookies, but given the FX constraints of the day I've heard that was largely budgetary more so than anything else.
There might also be some lingering resentment that, for the longest time, the only post Star Wars offerings were a couple of television movies featuring, of all things, just the Ewoks. No mention of any of the rest of the Star Wars universe. Other than the Ewoks, it could have been set anywhere else. But I think a lot of it is that the Ewoks manage to overcome the Stormtroopers.
If we were talking a long, protracted campaign, then I think the critics would be right. There's no way the Ewok would be able to withstand a coordinated campaign. The Empire isn't the Americans in Vietnam, after all. Assuming the planet was worth the effort, they'd wipe out the Ewoks in a heartbeat. Superior numbers, superior technology. If all else failed, they'd just vaporize the planet. (Which I suspect would have happened the moment the Death Star was finished anyway.)
But that's not what happens in the film. It isn't an entire war. It's one battle. Against an indigenous, obviously prepared guerrilla force. Yes, it's a force of three foot tall natives who look suspiciously like a marketing ploy. (It is George Lucas, after all, a marketing mastermind... to a certain extent... and it is Star Wars, which changed the movie marketing game forever.) Yet they know the terrain, they've put together various defensive/offensive efforts that are clearly aimed at the occupying forces - unless there's some T-rex sized predator roaming around that requires the smashing logs suspended from trees - and they aren't entirely alone. They have the Rebel squad assisting them, too.
As someone who's studied military history, I know this isn't the first time a smaller, less-armed force has managed to defeat a large, more technologically proficient force. Little Big Horn comes to mind, among other instances. Guerrilla warfare works for precisely the reasons the Ewoks manage to put a dent in those shiny white uniforms. Smaller, more mobile force, that knows how to use the local terrain to their advantage. In the long term, against a more ruthless force willing to use all the means at their disposal (which most opposing forces aren't - hence the reason we didn't firebomb North Vietnam into a wasteland) those advantages can be countered and overcome.
So, again, in the long term the Ewoks would be toast. Fuzzy toast, likely with the smell of burnt hair which, if you've ever smelled it, is highly unpleasant. But for one, short, pitched battle, with the element of surprise and advanced planning, there's no reason why it couldn't have gone their way. They might have even won a few battles before the Empire razed the forest and hunted the little pseudo teddy bears into extinction.
Hate the Ewoks if you must, but don't begrudge them their victory.
Although if Lucas ever comes up with a Jedi Ewok, then I, too, may be on the anti-Ewok bandwagon. Or would that metaphor work better with a Sand Crawler?