Bend, don't break?!
Is the extra load capacity due to lighter Al frame material?
I don't know anything about those trailers but some trailer and boat designs seem to have started in someone's garage. A guy with a welder can make a trailer frame or boat but that doesn't mean that stress analysis or endurance/reliability testing has been done, nor use cases defined. Maybe a bigger issue with boat hulls that take a beating on waves though.
I would be less concerned with material than the design, analysis, and testing behind it. I've seen industrial steel frames fail miserably.
I'd ask a lot of questions about the analysis, testing, frame racking, and suspension if I were shopping.
