That's really an unfortunate experience you had with your gorilla carts you have... I can't help but to think that you must have a different model than the one I mentioned in my posts above... I saw many users posted different gorilla cart models. I would not want to load them to full max rating (1200 Lbs) as like you said, I doubt they could handle that for long and they must have tested this in a non realistic environment. The model I have, apart from adding air to the tires has not given me any problem (My tires have a tube, so even cracking on the tire wouldn't hurt if it happened). I have it and used it extensively for the last 4-5 years. I actually bought a second one when I originally posted the sale at the time.
If it's made with some metal, you definitely can expect rust on those parts after some years (my climate is on the fairly dry side). I use it mostly for mulch, so not the heaviest out there, but this weekend, I was using it to load some grass sod with super dense and heavy clay soil. I used it also loaded with rocks and other heavy materials with no problem. The gorilla cart loaded as high as I could manage was so heavy that it took a bit of effort to start moving it on a slightly sloped ground, but it handled it really easy. The way I see it, with the model I have, there's no way I could ever even load the max (1200lbs) and then be able to move it. It's just too much weight.
This is the one I have:
There are a few bad reviews, so I guess sometimes you can get one with a tire problem out of the box, but overall, people seem to like it quite a bit. Btw, I did lose the pin on the handle once at the beginning, but never happened again...
Anyways, just thought I'd post this in case the model I have can get you to have a better experience. I'm also like you, I like to build things heavy duty so they last like a tank, I just wonder if someone could make one of those that is not too heavy with much thicker gauge metal. This is the reason why they use a really thick plastic cart to save on weight.
All the best