The removal of (poly)ubiquitin (Ub) chains at the proteasome is a key step in the protein degradation pathway that determines which proteins are degraded and ultimately decides cell fate. Specific polyubiquitin linkages may lead to distinct cellular effects, with several linkage types associated with proteasomal degradation and others with lysosomal degradation, protein cellular localization or signaling events. Three different deubiquitinating enzymes (DUBs) are associated to the human proteasome, PSMD14/RPN11, USP14 and UCH37/UCHL5. The functional roles and specificities of these proteasomal DUBs remains elusive. To reveal the specificities of these proteasome associated DUBs, we use SILAC based quantitative ubiquitinomics to study the effects of CRISPR-Cas9 based knockout of each of these DUBs on the dynamic cellular ubiquitinome. We report distinct effects on the ubiquitinome and the ability of the proteasome to clear proteins upon removal of either USP14 or UCH37, while the removal of both simultaneously suggests less redundancy for these DUBs than previously anticipated. We also investigated whether the small molecule inhibitor b-AP15 has the potential to specifically target USP14 and UCH37 by comparing treatment in wild-type versus double-knockout cells. Strikingly, we observed broad and severe off-target effects, questioning the alleged specificity of the inhibitor. In conclusion, this work presents novel insights into the function of proteasome associated DUBs and illustrates the power of in-depth ubiquitinomics for screening the activity of DUBs and of DUB modulating compounds.