I used UberX extensively during my holidays on the Croatian coast this year (Split & Dubrovnik), and while the cars are mostly nothing to write home about, the system works really well, there are flat fees to respective airports and virtually every aspect of the ride is better vs. local taxis. Even though I spoke with drivers in the local language, most of them seem to have a decent grasp of English, so no worries there. Customer complaints/feedback are processed locally (I presume in Zagreb), and the response time is very good.