I'm an active user of Fabric and have found it rather convoluted and limited. For example, there is no way to easily get device UUIDs from Beta testers. Given they also sponsor Fastlane, I'm blown away they haven't provided a way to automatically register UUIDs. This is possible with HockeyApp through their API and some Fastlane scripts.
Crashlytics is their killer feature. The rest is mediocre. Hopefully Google will improve it. I'm not holding my breath.
You might be interested in buddybuild's beta distribution which does automatic UDID management.
It basically makes it so neither the developer nor the tester ever have to worry about UDIDs ever again. Instead you can send a build to a new tester & trust they can immediately install it.
Crashlytics is their killer feature. The rest is mediocre. Hopefully Google will improve it. I'm not holding my breath.