I sometimes ride in a flat halter when I'm lazy. Just snip the lead rope on either side and ride like a normal bridle. You don't have much stop and it's very easy to brace against, so I would only ride like that on a horse I was confident would stop in it. You turning also isn't as great as in a normal bridle, but depends heavily on the horse what kind of response you'll get.
I regularly ride in a rope halter. A slightly different beast. You don't ride a rope halter like you ride in a bit. If you know how to ride in one, most horses are pretty responsive. A rope halter is "harsher" than a flat halter.