I think part of the problem is:
a) we've got very lucky with the likes of McGoldrick and Clarke coming in and hitting some of the best form of their careers leading to high expectations.
b) more scrutiny of our signings than any other club. Generally if a player comes in and doesn't do well elsewhere, unless it's a huge high profile signing, we don't really hear about it. Obviously we pay more attention to players coming here and therefore spot all the mistakes and think we have a higher proportion of failures than elsewhere. I don't actually have any evidence to back that up just a theory I have.
It is slightly worrying how quickly some players fall by the wayside when they come here, you could view this as a good thing - that SP makes up his mind quickly and doesn't persist with those who aren't up to scratch, the flip side of that is why are they being signed in the first place.