I've been with the I-O-M Steam racket on a few occasions,never had a bike damaged but,they put the bikes in "pens" then rope them together!! If one bike falls they all fall,also you're lucky if you get any protection from the ropes!!
Generally not as good as P&O,but better than Sea France!!
Not wishing to worry you,but there was a famous incident a few years ago when one of the ferries hit the dock!!! It looked like a war zone,thousands of pounds of bikes all on their sides!!! Heart breaking!!!