This morning I watched two videos on match defense.
In the first one ZAN tells how he played 42 H2H games with actual MLBs (Mike Singletary & Luke Kuechly) at LB instead of OOP safeties (Ed Reed & Jessie Bates). He found out that Singletaty & Kuechly (both with Lurker ability, of course) intercepted the ball at a higher rate than Reed & Bates, and the whole defense felt much more solid. Probably because they were playing their natural positions. There's a theory that OOP players take a hit to their play rec, awareness and coverage stats.
In the second one King Reggie created custom versions of Tyrann Mathieu, Jalen Ramsey, Darius Slay and Lavonte David - all 6 feet tall - and tested them in the practice mode. First he tested the D-backs at safety. Mathieu had the best skills, played his natural position but was the least athletic. Slay had the worst skills but the best athleticism. And Ramsey was somewhere in between. In the end: Slay did the best. He was the best in covering all kinds of routes, by far. Then Reggie tried Mathieu & David at LB. And again - the more athletic player, even though OOP, was clearly the best.
Obviously, there are more variables to this equation (play action, run defense, helping your teammates etc.) but still I was a bit shocked by the results: it seemed like the speed was what mattered the most.