I use them quite a bit.
I've reported wrong food orders and cancelled because of length of time.
One thing that really bothers me is the general lack of English speaking with the deliverers. Whenever I've called them directly, I've never understood them and they don't seem to understand me, perhaps deliberately.
I ordered Nandos once and I received a message from the deliverer. I looked outside of my flat by the main road to see her riding around. I called her, waved at her with half of my body out of the window, then after giving the clearest instructions that I could, repeatedly, I had to go downstairs, with an injured ankle, onto the main road, only to see that she's gone, without giving me my food.
I don't have anything against immigrants, and I get that people are trying to make a living, but you should have to be able to speak English on the phone when dealing with customers...