Horizon is basic compared to OW or PA but its fine, clean and comfortable. Food is fine as well and not that badly priced imo.
Motorhomes and trucks will travel on deck 3 which is a lot more sheltered than the top deck which is only for cars.
That said there are some ventilation holes on the truck/MH deck which can let spray in but not in a huge way, we were parked next to the holes on our outward crossing which was 4-5 and here was some spray on it but nothing major.
Cabins are clean and modern, everything worked and they are well ventilated.
I'd use that service again no problem, especially if its 500 cheaper than the BF Pont Aven.