Hi I'm not 100% on this so don't shoot me if I'm wrong, but I think some of the reasons that Posiedons are frowned upon is because..
1) They use non standard hoses and fittings, and therefore do not conform to the standardised equipment protocols required within a DIR team.
2) Some of the regulators used to be (I'm not sure if this is stilll true) of an upstream design meaning that if they failed then they failed shut.
3)Having a yellow shower head stuck in your gob was a massive breach of rule six.
Although I think that number one is the only relevant one these days
