I've got a set of the grey colored, heavier duty Rhino ramps. I've used them many times, and not seen any cracks. The Camry and Avalon are the heaviest vehicles they've held (well, I put the neighbors mid-size SUV up once too).
Irrespective of the integrity of one's ramps, I strongly recommend always having a secondary support under the car before you go under there, for any reason, for any length of time.
I've mentioned this here before, but years ago, I represented the family of a U.S. Navy Chief Petty Officer in a claim for life insurance benefits (why the need for an attorney is another story). He died because he tried to do work under his Ford using only a tire jack!!! The poor guy died because the jack failed and the car sagged on him, compressing his chest and suffocating him to death. It's been over ten years, and I still wish I could push a "reset" button on my head to erase the memory of reading the description in the autopsy report of how this man died. He struggled for a good minute or two, fighting a losing battle to breathe before the overwhelming mass of the car overcame his will to live.
Anyway, I don't even think about going under a car unless I have TWO separate, independent lift systems in place. Usually, I just slide a large, stout metal tool box under part of the body structure so as to allow me escape space just in case.
It may be a low-probability possibility, but do any of you want to spend your last minute or two on earth wishing you had ensured a secure lift (while knowing you're suffocating to death)?