Speaking from the Doordash customer side, I don't think I've ever had an order not be accepted. If there's an issue, you can either do some automated complaint via the app and get automatic resolution, or you can talk to a live person. Generally the easiest is just to get credit, though I think if you talk to a person, you can get the money back on your card (maybe you can through the app version too, not sure).

I've never had a claimed delivery that didn't happen, but I have had wrong deliveries. Usually just some piece missing (like a drink or the like). Once I got someone else's order in addition to mine, though I made sure when I contacted DD I pointed out it was the restaurant's fault (printed my name on all the items, even the wrong ones). Once, I got the entirely wrong order, so they refunded me my whole order (and of course I got to keep what they delivered, not that it was very great but hey, free food I guess). It was through that that I found out that the App can group orders together from the same restaurant, which is why sometimes you'll see the driver seemingly going off in the other direction.

Also, I'm pretty sure the DD extra tip goes 100% to the driver. They have a suggested amount based on how hard it is for the driver (like distance and number of items I think), not a percentage, for my typical order they suggest $5 which I'm fine with.

Anyhow, TL;DR, it works well enough most of the time.