I've done it once, and once only - was at a large (but reasonably up-market) farm shop. The app simply didn't function properly - the sign-up process was broken, such that it was an endless loop of emails and failed requests, to the point where the service manager had to spend 15 minutes sorting it out. We then spent 15 minutes placing our order, because the app kept clearing our order every 5 minutes, but was so slow that we couldn't complete it any faster (no wifi, and barely any reception in the restaurant). By the time we'd finally got an account sorted and our order placed, we'd been there 45 minutes already, and it was only then that we were told it was going to be another 45 minutes for the food to arrive, which turned out to be an hour.
Of course, there was no way for us to get updates, because they clearly thought that forcing everyone to use an app that barely works meant they only needed a skeleton staff - 5 waiting staff and a service manager for over 300 covers, at lunchtime on a Saturday.
The food came out, and there was bacon in a dish that was clearly labelled "vegetarian" in the app - another 10 minutes to get hold of someone, and "Oh, yes, we changed some of the menu but the descriptions haven't been updated yet". Send it back, want the other dish re-cooked so I'm not eating cold food..."The kitchen's quite busy, it'll be about an hour". Could've been avoided if we'd actually been able to ask somebody at our table to verify it was suitable for vegetarian.
Order cancelled, asked for a refund (apps make you pay up-front)...another half hour because the manager was dealing with three other parties in the same boat.
Two and a half hours later walking in, we finally left without eating.
Never again.