On the issue of Gungans, I wouldn't say they're MORE primitive than Ewoks... I wouldn't even say they're as primitive as Ewoks actually. Gungans had utilized mechanical transportation that, for underwater, was as advanced as air and landspeeders on the surface really.
They'd also formed modern-ish looking cities... Or so it seemed, there's not a lot seen of Otoh Gunga.
Also their weapons are energy weapons... Like the weapons of the Geonosians, they are unique to the species, but nonetheless they are a HIGHLY destructive energy weapon when used against the Battledroids. I think the bigger issue with the Gungans is they are, really, nothing more than a simple diversion...
For instance, the Gungans weren't confident in their shields, they knew they'd fail eventually, but they still went into combat. There was distinct mention of the losses though... Nass said they'd do what they had to though and all and they did.
The other important thing is remembering that the Federation knew they were there in the swamps... It was a matter of time at that point before they attacked that area, so the human forces moved in to assault the city while the droid army exited that area and assaulted the Gungans... Really though, the strategy was pretty much as sound as it gets for all the more they had to send into the battle.
I definitely don't look at the Gungans as archaic though like the Ewoks... They're just different. Their shield withstood a bombardment even from some pretty hefty artillery. It only collapsed later into the battle when something hit the Faamba's back... It almost makes you wonder if something WITHIN the shield hit it more than the artillery blasts. The AAT's were sorta right there to move into the battle when the shield did collapse so maybe one of the droids went in with a warhead launcher or some such... It would seem odd for the AAT's to cease firing just to get the infantry out, then start again when they attacked.
On the issue of an AT-AT, there was one on Endor actually... At least one, if not more. I think it was far away from the rear bunker, closer to the main complex where I think the INITIAL plan of attack by the Rebel Commando team was focused... I think that's what made this whole mission such a "suicide" mission, whereas it became substantially "easier" with the back entrance they didn't even seem to know about.
The area around the complex, IMO, was probably a wider open space, with massive deforestation. In shots of the model for the Endor Shield Generator complex there's 3 (or 4) heavy artillery towers like on the Death Star's surface, the generator, etc... The massive explosion implies it's a fair distance from the back door to the generator complex too of course. I figure the area around the generator is pretty well leveled and flat, good for an AT-AT to patrol the perimeter or whatnot... The rear door complex is patrolled by scouts and AT-ST walkers, though the ST Walker still has trouble with the heavy forest really. In video games it's not terribly difficult but it does lend itself to easier attack on the walkers, which I think makes some sense.
So to me an AT-AT coming into the battle as we see it really isn't going to be effective. It won't be easily destroyed by log traps though of course, but I think it wouldn't do much damage to anyone either... It'd be shooting down into tree trunks at targets it couldn't find. It also could be quite a distance away waiting on an attack that never comes at the main complex...
I have this theory that the forces at the back door are the weaker element, and the forces at the main complex are still expecting a heavier assault there of some sort. The forces at the back door are definitely not what I'd consider "an entire LEGION of my best troops" though. They seem like a hodgepodge mix of Naval troops maybe working the station more than being part of the legion itself, and scouts with some Stormtrooper support and the walkers that maybe HAPPENED to be somewhat nearby for whatever reason.
Just some thoughts on this topic additionally... Endor's an often debated one I've noticed, always a good topic to discuss.