It depends on the device. As I am not a developer - I am an engineer, however - I presume there is an issue related to the read-only system partition on certain devices which prevents built-in apps from upgrading during the ART run time switch over process. Therefore, upon reboot, the system attempts to recompile the apps for the new run time without success. Once the software has been updated to 5.0x or 5.1, the developers will have recompiled the built in apps to support the new run time, and user installed apps will continue to function as expected.
My 2013 Droid Maxx was able to run ART without issue. Frankly, if I could have my Droid Maxx, with the 1080p screen from a Moto X 2014, I would have no need for the Turbo. The 4.4.4 software on the Maxx was stable, quick, and responsive. And, the Maxx had VOLTE, as well as simultaneous voice and data without VOLTE enabled. The Turbo is a great device, but much of the horsepower seems excessive for business use. And, some of the system changes made from its predecessor seem like a regression. However, I am continuing to use it with success.
End of rant. ;-)