mvdemarco,
The frame rails do look a little fragile, but over 5200 rounds through my USP .45F since this past June and I have no complaints. Why, because I've had no failures of any kind. Gunhead has twice as many rounds through his USP and he sounds like a satisfied customer, I'm a satisfied customer, and if this is your only concern with the gun I think you'd be a satisfied customer also. HK didn't win a government contract for the SPECWAR guys because of cheap construction. Check out the link New_comer identified, it will give you the info on how the USP MK 23 was tested. Also, grindliner makes a good point the about all USP having the same lifspan. The same manufacturing process for the MK 23 is used for the full line of USP's. Also, HK states in the USP manual that their guns are rater for +P and +P+ ammo without any modification to the gun. All this being said I think it is safe to say that the frame rail tabs will hold up without any problem.
grindliner,
Depot level maintenance? NSWC Crane? You work in the navy engineering field? I work for NSWC Carderock in Philly, I'm a Fleet tech rep. I don't see anyone talking about depot level maintance on the boards here, or anywhere. I'm just curious.