This does not mean that a round reserve parachute is dangerous. Just that you must remember the limitations it puts you under. As you descend under the parachute, air is forced out the back of the parachute through these slots, pushing you forward. You steer by pulling down on the rear risers. If you pull down on the right rear riser you close off the slot on the right side of the parachute.
More air goes through the slot on the left, pushing the parachute around to the right.
The rate of turn is much slower than a square canopy. Also, the round parachute cannot be flared for landing. The slower forward speed means your spotting and landing pattern are much more critical. The cone of maneuverability under a round parachute is very narrower compared to a square. If you are off the wind line, you need to start crabbing right away to get back to it.