I've used Uber in Bogota, Madrid, Amsterdam, and Paris.
I've had about 9 good experiences, and one bad.
Here's the issue: while technically you just need data/WiFi for the hailing, I highly recommend some scenario where you have data for the whole process.
Before I started using SIMs/buying data -- basically, the days when I only had WiFi - I hailed a cab from the lobby of the Park Hyatt Vendome in Paris using their wifi. Once Uber said "we're on our way, see you in 3 minutes" or whatever, we left.
We sat, and sat, and sat....nothing.
He's calling, we can't find him, blah blah blah.
Next thing we know I'm checking my AMEX statement and see a 45 euro charge...for thirty minutes or so of "waiting" - ha!
Über refunded me the money, but refunded it in euros, so I can't use it until next time I'm back in EU.
The reason I say "get data" is because, if we were connected the whole time, I would have been able to see his GPS coordinates and I would have known where he was.
Furthermore, I've had friends who weren't connected to data and the uber driver took them on the most convoluted/fee heavy route imaginable. Being able to track this on the phone would have been a huge perk.
So, yeah....doubt you "need" data for anything but the hailing...but now I make sure to have data so I can watch where driver is/where he's going/etc