While in UK, I've only used just eat for kebabs from a turkish ta, the only reason being that (1) I was starving (2) I had no cash on me and just-eats accepts card (fees apply, etc.)
With my move to Ireland, I use Just Eats quite regularly but I can't say I've ever had a truly remarkably b ad experience. Perhaps I was lucky and picked the right restaurants? I surely remember going through reviews, but Cambridge in general is filled with pretty decent, all around BIRs. Getting your order wrong is a common theme that happens in a lot of these TAs. I always call them back as soon as I notice something missing and ask them to deliver it ASAP and ignore their plead to have it as credit for the next time. One thing worth highlighting is that if you want food done in a specific way, don't rely on the field where you can add additional notes to the restaurant. My experience is that the staff ignores this. When I want something, I always call the TA as soon as the order appears as "accepted" on the screen, to highlight it, because then if it comes wrong, I will give them a call back to ask for a new order seeing as they've ignored my request.