If the knot is tied right, they won't come loose if the horse is straining on em. I had an old mare one time that lost her cool while tied up to a hitching rail one day. She was sitting back for all she was worth and when the buckle on the halter broke she cut a flip straight back.
She was a crazy old thing anyway.
I use a rope halter on our gelding when he needs it. He "ain't " going nowhere with it. I switch with riding though because it doesn't fit well with the briddle.