For one, there seems to be some variation on what his spin rates were in college. Some clocked his slider in the 3000's, which would make it one of the better sliders in baseball if that were true. But others clocked his spin rates in the lower 2000's, which makes it a very average pitch. So far, the data in AAA and and at the ML level suggest his spin rates on his slider are closer to the mid 2000 range, which makes it very average. But his splitter garnered more swing and misses his junior year of college than Skenes slider did. So that itself shows you it is a really good pitch.
The main culprit that holds him back and was most certainly a knock on him in college, is that he has bad command. His walk rate of 5.0 as a Junior is pretty clear evidence of that.