Esri seems to have dropped support for Collector on Android devices running OS v. 5 or lower, or there is a bug that primarily affects those devices. Our organization is getting sync failure on several makes and models of devices running Android 5 or lower, while devices running 6, 7, and 8 are able to sync. Also, one of our volunteers reports that Collector is not available in the Google Play Store for his device, which runs Android 4.0. This is troubling since Google reported in December that nearly 30% of active devices are running Android 5 or lower.
Also, Collector routinely crashes after about 10 minutes or less on my Android 8.1 device (Pixel 2XL). Our organization has been experiencing difficulty using Collector on Android devices in general since we began using ArcGIS, but since the software is ideal for our application we have tried to deal with the problems as best as we can, which is hard when asking volunteers to deal with the frustration.
We have found Collector is fine (on-line and on-device) on Samsung and Apple. But only works on-line on Pixel 2XL and HTC. When you try to attach a base map to take a map off-line on Pixel 2 and HTC it crashes. Hopefully the new look Collector will be fine on all Android devices when it is released in early 2019.