Sorry, I've been on the road for the last month and just realized I never responded to you. My bad.
I have yet to find stress fractures or cracks on my shell. I've had the shell almost a year and the tents been up there for well over 6 months with lots of off-road use.
The only issue we had was we were going up a long trail that was up an incline for a few miles and when we parked the shell had actually slid back about two inches on the bed rails. I loosened the clamp mounts, slid the shell forward, tightened the clamps and we carried on. I have since had holes drilled through the shell and through the tops of the bed rails so there is no chance of that happening again and we haven't had any issues since. I looked at the other brands and they all mount the same way so this could be an issue with ARE and Snugtops as well. Just something for you to keep an eye on if you get a shell. You can see the shell remount at my build thread at nobody.tacoma3g.com
As for weight ratings/limitations etc, you won't find them. All the manufacturers of the racks/shells etc will try to pass the buck to the other guy instead of give you a straight answer. The shell manufacturer will tell you the shell can handle what the rack can, and the rack manufacturer will tell you the rack can handle what the shell can. It never ends and no one really gives you a straight answer when it comes to dynamic versus static load ratings. To make it more complex the various racks on various shells make for infinite combinations of potential load capacity's that each manufacturer isn't interested in figuring out for you.
What I found, Leer has and should warranty damage to a shell caused by a loaded rack with a maximum dynamic weight on top of the shell of up to 250 pounds. That's directly from my dealer.
Prinsu doesn't really give any kind of load ratings. I chose the Prinsu rack for me though because on the long bed shells the rack is bolted in 6 separate places to the shell. In my mind this could disperse the weight on the rack to more locations across the shell. This should in theory spread the load more evenly across the shell structure. Whether I'm right or not only time will tell.
Hope that helps and sorry for the long winded rant. ??