I can't speak specifically to the Volt, but I've owned a few aftermarket warranties. They can be a decent tool, but you have to shop carefully. In general:
- Aftermarket warranties that are branded by the manufacturer are better. If it's branded "Audi Protection", then even if Audi isn't the one paying for stuff, their brand name is on the product and they have influence with the warranty company to keep them at least somewhat in line.
- Don't expect to make money on the warranty. You are buying insurance and like all insurance the goal should be to manage risk. The warranty company has done the research, they know what the average cost to them is going to be for a particular car, and they're going to charge enough that after all costs (payouts, admin costs, etc) they will make a profit on the average car.
- Never buy a warranty with a list of things that are covered, buy one with a list of thing that are *NOT* covered. This is where the weaseling thing comes up, there are so many parts in a car that it's very easy to claim that a failure in something that's listed as being covered is caused by something that isn't covered and therefore isn't covered. The exclusion contracts (which are typically more expensive) are much better in that regard.
- With that said, if something is on the exclusion list, go in fully expecting that it will not be covered under any circumstances. For example, alignments aren't covered. Fine you say, alignments are maintenance, not repairs, right? Well, imagine there's a covered repair that requires dropping the subframe, and doing that requires realigning the car after reinstalling it. If so, you're going to be paying for an alignment in addition to your deductible.
- Look for a contract that has a pro-rated buyout clause. If it's a 4 year contract and you're 2 years in, you want to be able to cancel it and get half your money back. This is important in case you sell the car or it gets stolen or wrecked, but it also gives you some recourse to get some of your money back if you later decide the warranty was worthless. Here in CA, clauses like this are required, so they all have them. I'm not sure how common they are in other states.
So with all that in mind, personally I wouldn't buy an "electric drivetrain only" warranty.