With Cosart, it was less about the talent he was facing and more about the fact that he could barely hit the k zone. He had an ungodly 7.48 bb/9 in 2017. It's not like he is lighting the world on fire in high A as a 24 year old since he's been sent down either. It doesn't help that his velo has dropped 3 mph.
From SP: Summation: Potential to develop into a seventh-inning reliever if he can continue to refine command and control and develop a secondary offering to compliment his fastball. Unique mechanics provide deception, but also put a tremendous amount of stress on his arm and as a result, greatly increases his injury risk. High-risk prospect with low floor, but some upside if things work out.
Also, Cosart has a brutal, max-effort delivery which features no incorporation of his lower half or extension, instead relying on his lightning-quick arm. Even when he is on, Cosart can have trouble throwing strikes at times.
Hard to really compare Feltman to him at all.