like 100%? probably none for aftermarket, i bought an aftermarket warranty for my LLY when i bought it with 95 k on the clock, the warranty was 3100$ and 3 year/36, the company was VehicleOne. the head gaskets blew, and it had alot of leaks, front diff halves, front and rear axle seals, and pinion seals, the end bill was 8900$ i paid 2500 out of pocket, 750$ of it was head studs, because the warranty wouldn't cover it, as well as some machine work, and they would not cover new heads, bare or assembled. warranty paid for itself the first time i used it, they also covered some more axle seals later on, they were pretty easy to deal with. and they did not question any of the aftermarket stuff.
these guys were the first decent expierence ive had with an aftermarket warranty, i only purchased the warranty because i was buying a LLY and figured the head gaskets would go at some point soon.
hope this helps