This may not be for the squeamish...
I don't know if it's called vegitation (vegetation?), or if it's a separate issue, but I have seen photos of mechanical valves with the owner's tissue growing on them. Apparently, the body strives mightily to cope with whatever is introduced into it, and occasionally it can manage to take root even on the inhospitable, madly gyrating surfaces of a mechanical valve. The appearance was that it was not near the edges of the sealing surfaces, but rather on the flat face of the valve.
This is just FYI. There was nothing with the photo to state whether it was particularly detrimental or not. There was no discussion around the circumstances for which the valve may have been explanted. As such, don't read anything into this that isn't there, or draw any unpleasant conclusions that that we have no facts to support. I have never read of this actually being a problem to anyone.
I suspect tissue valves must acquire growth more often than mechanicals. However, it is calcification, not tissue growth, that is credited with damaging and finally ruining tissue valves over time.