I just looked at my Rhino ramps and they are the 12000 lb labeled ones, they have the honeycomb bottom meaning there is no voids larger than bout 2" under there.
That sounds more legit maybe I'll check those out. The lighter duty ones shown earlier don't look beefy enough to make me comfortable using them.
Daves69 having to jack one wheel up at a time kinda defeats the purpose if I had to do that I'd just put it on stands in the first place. You might try putting some kind of sticky rubber-like material on the bottom of the stands to hold them in place on the ground. Also that part you pointed out not having welds, it really doesn't affect the strength of the ramps anyway all the force comes down the vertical sections to the ground. As long as it doesn't bow out to the sides when you put weight on it it's fine.