Voice AND data (and SMS) work fine on European networks.
I took a stock Verizon Moto X to Rome, Italy. No unlocking, no rooting, no special side-loaded apps; just a plain vanilla Moto X. I pulled out the Verizon nano SIM and plugged in a T.I.M. (Telecom Italia Mobile or something close to that) nano SIM. It just worked! (Note: you'll need a paper clip or earring stud or something to pop the tiny SIM tray.)
When I boot the phone with the foreign SIM card, it first asks for a 4-digit SIM PIN. This number is printed on one of the cards from T.I.M. Then the phone puts up an annoying message: "Sorry, this SIM card is from an unknown source". Then it goes to the home screen, and all is good. Two small annoyances: you have to enter the 4-digit SIM PIN every time the phone boots (you get 3 tries at the PIN - after that I don't know what happens); and it seems to want a reboot about every 2 or 3 days - the symptom is data seems very slow or gone, but a reboot (with 4-digit SIM PIN) makes it all good again.
In the place along the top notification bar where the phone would (in the USA) display the "4G LTE" logo, in Rome it would often display "H+", presumably indicating some kind of HPSA+ connection. I know nothing about European signaling standards, but presumably H+ is good.
We used voice and maps pretty heavily, for example, speak the command "navigate to the Borghese Gallery," choose walking, and you're on your way. Mostly it could understand my english names for places: the Pantheon, the Vatican Museum, the Trevi Fountain, etc. If I had an Italian street name or piazza name, I'd have to type that in (for example, it never understood the voice command "nearby gelato" or "nearby gelateria."). On the other hand, commands like "find nearby ATM" or "find nearby artist supply store" worked pretty well. Plus we were always using voice commands to ask google random questions like "list of the Caesar emperors," "when was the Pantheon built," "when did Bernini carve Apollo and Daphne" and they all worked fine. Finally, it is useful to have a local phone number when you are making restaurant reservations or scheduling your line-skipping tickets to the Vatican Museum or your special guided tour to the underground and 3rd ring levels of the Colosseum. So a local SIM card has real advantages.
Oh, I almost forgot, data usage: under 220MB over 8 days. We usually did email over the hotel wifi network, but most of the maps/guidance and random questions were on the T.I.M. data plan. So for our purposes, a 1GB data plan was more than enough. Obviously your mileage may vary.
Happy travels!
- elwinc