[Elecraft] What makes K2's SSB performance less impressive than on CW ?

[PE1E]

I bought a K2 for SSB use ( + DSP option ).

In various reviews ( e.g. EHAM ) I noticed that K2's CW performance can't be
praised too highly, but there is a bit reserve as to the SSB performance.

In which respects - if  at all - lags the SSB performance ?
And what would cause this ?

Thanks for any comment.
This is a great reflector.

Peter, PE1E.