Unless it was optional, it might be less than ideal for people running more the one device.

I have a primary tablet which records every flight, and a secondary one which is sometimes, but not always deployed, and which will usually be put into flight mode at some later point in the flight. I also have a phone which I use only as a traffic display, and is only logged into SD when I update the base charts and an AIP or two. I wouldn’t want it flooding the cloud with duplicate flights, often with the wrong aircraft, every time I log it in. 

There are some things best left to user control, especially when the manual sync method is so trivially easy. 

If there is an auto sync, please leave an option to switch it off as it will screw up my logs, guaranteed.