So long as you stay between 0 and 45 degrees you should be fine, but anymore than 45 degrees you risk a nasty back injury in the event of a big off. The force of the accident pulls down on the harness instead of across it, if that makes sense.
Also, in the event of a roll, if the roof crushes and you are strapped upright the only thing that will stop the roof crushing is your head. When using normal seatbelts you'll roll forward, which is impossible in harnesses.