Here in Dubai - they are ludicrously incorrect. I'll see a "8 minute ETA" that turns out to be 35 minutes, more often than not.
This seems to be a very solvable problem. During certain hours, traffic is very slow. I've only been here a month, but I can tell you at 3:30 - 4:30, you need to add about 15-20 minutes if you are going near the interchange at Wafi Mall - That's without even taking into account the various traffic flow indicators that I'm sure Uber has access to.