Since my post back in March, I have used the OnX phone app on two phones for about 1 1/2 months. OnX is still a bit short on features that I need but as far as actual use in the field I found the following..
My Iphone has limited memory and was used for the initial testing. I found it to work error free "provided" I took certain steps. I turned off cellular data for the app, always placed the phone in airplane mode, then opened the app. Closed the app then took the phone out of airplane mode for other uses. I only used wifi for downloading maps and as stated, never used cell data. I found cell data caused the app to be extremely slow and occasionally appear to lock up.
Once I saw what I felt was needed and realized the memory problem I had, I grabbed one of the kids old G3 out of the junk drawer. It has a much bigger screen than the iphone5 and has 32gig internal memory. OnX will not write to an SD card so that memory was a big deal. This G3 is locked to a carrier we don't use so is wifi only. This phone has worked flawlessly from day one. I turned off all cellular data capability I could find, used wifi when downloading maps, and otherwise, the phone lives in airplane mode. If I download a map, I close the OnX app, turn wifi on, then open the app again.
On our archery hunt we biked an average 20 miles on our morning hunts and 5+ on the evening hunts with tracking enabled. Battery usage was increased but the tracking feature was within a few feet. The direction of travel arrow worked well too but we didn't use it too much.