His ear for music / beats is not sharp or discipline to be the goat.
He'll let bullshyt or unmastered beats on his albums if he thinks they're cool enough or he can ride on them pause.
You see that in the quality of beat selection on parts of SDE and all of his work after diplomatic immunity. He'll settle for good enough, let a beat clearance issue fukk up a song and accept a lower quality replacement beat, or choose songs that are way too muddy or loud or poorly mixed, because he probably thought it sounded cool in the studio or he has some clever lines, and nobody would correct him.
The result is you'd get great mixtapes because there are no rules and lower standard, and you get uneven albums with clearly bad songs on them or songs that are 70% done sounding because Duke da god was anything but a disciplined AR.
Rhyming skillset, I suppose but I'm not gonna say he got bars or songs of rhyming like Nas, cool j, rakim, etc.