Whether or not I will hand-feed depends entirely on the individual horse. My own fella has excellent ground manners, and will back calmly away from his feed pan mid-meal if I so much as glare at him and "reclaim" that food as mine, so he gets a cookie by hand a few times a week. He's never had a problem getting pushy or snatchy about it, and has never offered to develop one in that department. Still, if anyone who doesn't know the drill wants to give him a treat, they get the rundown of how to do it safely.
If the horse is struggling with basic ground manners and respect, and/or is in the habit of being pushy or bargy even when treats are not involved, I won't hand feed. I'm all for giving treats every now and again, but I'm even more all for not being mauled over it. At the end of the day, hand-fed treats are a luxury, not a requirement. I can reward good behavior easily any number of ways without reaching for a carrot.
As far as carrying food into a herd situation, I'm quite a bit more guarded -- throwing other rambunctious horses into the mix can make even the most gentle horse forget himself.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown