I use the slipway in Ireland referred to above - it is nothing like this one (which actually looks quite OK in the pics). The slipway here was covered in many months of full on seaweed growth. So long as the vehicle tyres have traction then IMO, a slipway is useable (with care).
I'd caution about complaining too loudly to the Authorities. They have a tendency to simply close "dangerous" facilities under their care rather than remedy them. Our local Council simply locked the offending slipway and forgot about it. No more launching boats in Buncrana!
Chlorine is now banned for slipway cleaning - it has a fairly wide killzone and takes out a lot of marine life as well as the seaweed - especially in enclosed docks. It's very effective though and mysteriously, our local slipway never seems to get much seaweed buildup. A lad slipped and died as a result on the same ramp a couple of years ago and there was a lot of muttering in the local community. The weed just sort of gave up after that...