What sonic artifacts are caused by phase vocoders?
The term "phase vocoder" may encompass more than one resynthesis method.  One method uses a pruned frequency reassignment of the DFT basis vectors, and thus resynthesizes sound using a sparser set of basis vectors with slightly different frequencies than were present in the original DFT analysis windows.

This sparse basis may not be able to recreate a full spectrum wide-band signal, such as produced by a transient, as well as the original non-reassigned full DFT basis.  

Phase vocoder analysis also makes the assumption that the spectrum is fairly stationary between FFT frames, an assumption that can also be violated by transients.  

One term for the artifacts produced by this type of analysis/resynthesis is transient smearing.