I have no experience from USB on OS4/Sam/A1
So you're basing your entire opinion on some threads where people had trouble. Of course, everyone who has had a great experience with Sirion probably didn't write about it all over the forums. Which is the problem with forming opinions about stuff you have no real idea about.
IMHO, Sirion is not perfect, but when you have supported devices, it works nicely and non-intrusively.
Sure Poseidon currently supports USB2 and some devices Sirion doesn't, but personally i don't think there is any great need for Poseidon on OS4. Sirion is improving all the time.
BTW, as an owner of an Algor, i can actually speak with experience about Poseidon. Poseidon is a great, well written, stable, quality product. But I don't miss it on OS4 at all.
Edit: BTW, the one time I did have corruption on a USB device it was due to a bug in an (OLD, and never publically released) version of CrossDOS, nothing at all to do with Sirion itself. I've never had any corruption with newer versions of CrossDOS, or when i made very heavy use of my external USB hard disk formatted with SFS.