I have rented vehicles, (and airplanes) for many years and have not run into too many problems, though I have had to put my foot down a couple of times when I knew I was being scammed. I usually rent from Budget, Avis, or National. Budget did ding us about $45 when my wife hit the curb and lost a wheel cover in Wales, (neither of us were used to driving on the wrong side of the road

). They also dinged us $75 in S'toon when I broke a key on a Ford Explorer, but I know Ford keys, since they are electronically coded, are expensive.
My last rental was an SUV in Calgary from National, I noticed several stone chips in the windsheild on my initial inspection and brought the form back in to indicate where they were, (approximately). No problem.
The other scam is the insurance they want you to buy, either your own policy will cover what they do, or else your credit card company will at no extra charge, but you have to make sure of this before you leave.
BTW, I busted two windshields in my Chevy 1/2 ton, they cost $190 installed, but the insurance company was billed $350, you wonder who's doing the scamming.