I hand feed my horse all the time. He does get a little pushy, but once he figures out I have no more treats, he gives up. I can see it becoming an issue for someone who maybe doesn't know how to correct the pushy behavior right away, but if you know what you are doing and you know your horse, it shouldn't be a problem.
Now, walking out into a pasture with carrots is a huge mistake, no matter how well you know the horses! One dominate nip from one horse to another could spark all kinds of dangerous behavior from the entire herd. Treats should be given occasionally and to one horse at a time, not an entire field full.