If its a factory warranty then yea they can be good.
The aftermarket ones are usually junk and cause more headaches then they are ever worth.
Yep, and dealers don't have to honor them, either.
We did that with a couple of the warranty companies when I worked at a dealership.
The ones that were known to be pains, we'd tell the customer they'd have to pay the bill and then deal with the warranty company. We would not even make a phone call.
I can't blame dealerships for doing that, either. Some of these companies will want the part # of every single thing you're going to replace. Then they think they're entitled to a discount on the parts. Then they'll want to argue with you about the labor time, sometimes even if it's right in the book. Then they'll want to argue the labor rate.
Then, if it's an engine or tranny, they'll want it torn down, and then send and inspector out to take pictures of it for them. So you have it sitting there, torn down and taking up a stall/lift, waiting for the inspector. THEN, after all that, and agreeing on a final price, sometimes when they call back with a corporate credit card, it'll be for less than you agreed with them on.
I use to say they just short it by some random amount to see if anyone will call them on it or if the customer just says "screw it" and pays the difference. Or the dealer eats it so they don't have to make another long-ass call.