Upgrading iOS on a device is always an unpredictable event. For many, updating will be fine. For others, a problem. Even when updating the same generation/capacity device.
Having said that, I think it is "safe" to upgrade the Air 2 to iOS 10. The primary reason being that the 9.7 iPad Pro has only 2 GB RAM and was released this year so Apple won't/shouldn't be too aggressive in pushing the need for RAM. I upgraded my 12.9 iPP and it went well, iOS 10 is nice on it. I upgraded my wife's Air 2 to 10... it went well and performs just as well as it has always done.
I would advise caution (caution, I'm not advising against it) when upgrading any device with less than 2GB RAM.