These are not really warranties, but a form of insurance complete with the usual fine print. By entering into the repair insurance contract, you are betting that you will have more repair expense covered by the contract than the value of what you pay for the premium. The company sets the odds on the bet, and then gets to tell you whether or not they are going to pay off when you "win" with a big repair bill. And, to make it even better for the company, in most states this is not regulated by the state insurance department--if you get stiffed, the courts are your only resource on what the contract means. Since the contract is written and presumed to reflect the intentions of the contracting parties, usually nothing in the verbal sales pitch can be used as an aid to construing the fine print.

All this being said, I did once have a head-off repair on a Benz that was more than the cost of the contract. I have also had a garage told that the "warranty" company just wasn't going to pay on a Volvo repair, even tho the repair was squarely within the contract.

Get copiies of the actual contracts from anyone you're looking at doing business with, and make your own estimate of the value of the bet against the house.
Good luck!