One good way to do this is review NFL draft information. It is the best way to assess talent level at the schools. It's not clear if it is recruiting, development, or both. Moreover, there is almost no way to determine the root cause of the effectiveness of talent acquisition: bag game, charismatic head coaches (Swinney > Stoops), etc.
Venables' final year at OU was 2011, so he had considerable contribution to folks drafted through the 2014 draft. Here are the OU defensive players drafted from 2008-2014:
2008 - Curtis Loftin (LB, 2); Reggie Smith (CB, 3)
2009 - Nic Harrs (s, 5)
2010 - Gerald McCoy (DT, 1); Keenan Clayton (LB, 4); Dominique Franks (CB, 5)
2011 - Quinton Carter (S, 4); Jonathon Nelson (CB, 7); Jeremy Beal (LB, 7)
2012 - Jamell Fleming (CB, 3); Frank Alexander (DE, 4); Ronnell Lewis (LB, 4); Travis Lewis (OLB, 7)
2013 - Stacy McGee (DT, 6); David King (DE, 7)
2014 - Aaron Colvin (CB, 4); Corey Nelson (LB, 7)
The defense was good, but not stocked with NFL talent.
I'l leave the deep dive into Clemson NFL draftees to someone else (or I'll look at it later), but here are some highlights
2016 - DE in 1st; LB, 2 CB in 2nd; LB in 4th; DT in 5th; S in 7th
2017 - CB in 3rd; DT in 4th
2018 - 2 defensive players drafted
2019 - 3 DL drafted in 1st round, 1 CB in 2nd round; 2 more from defense drafted later
2020 - LB, CB in 1st round, LB in 3rd round; S in 4th;
2021 - zero players from defense drafted
Again, I have no idea the root cause ($$, recruiting, development). However, I see the players at Clemson as being far superior to what he had at OU.