IMHO none whatsoever... I have seen errors of 10,000ft plus, but in fact I guess we are more likely to spot the gross error (which perhaps equally suggests there may be even more errors that slip by un-noticed?).
In one interesting case I was working traffic on the South Coast below 3000ft... when I got a call from Swanwick asking me to confirm the level of my traffic, which their equipment was picking up at FL320. The SSR had been verified and was indicating correctly on my display.