The bonus about IR cams is awesome battery life. You can get 3-500 pics a week and still last for 6 months. Try that with a flash.

Flash scaring the deer? I dont know about that. Depends on the individual deer I guess. The IR's(except high dollar black flashes) do lightly glow red and still go click when they take pics. To me, IR seems a little less invasive.

I did borrow a cuddeback flash from a friend this week to try to get color pics of a pair of albinos I have on my property. It is the first time I've ever really wanted a color pic.