If spin rates and pitch velocity were the only things that mattered, Pagan would be great. He's not. You can have the best spin rate in the world and the highest velocity in the world, but if the hitters know when and where the ball is going to be, they're going to put it in the seats. Very few pitchers are Mariano Rivera who can throw a single pitch with devastating results.
Deception is important. If hitters know what's coming, they're going to crush it.
Placement is important. Hang a curve in the meat of the strike zone and hitters will crush it.
Velocity is valuable. Obviously, faster means less time to react.
Spin rate is valuable. The faster is spins, the more it moves, but when it moves can depend a lot on the grip and throwing motion. If a ball consistently moves in a nice arc, hitters will crush it. Later break is much better.
Bundy may not have huge velocity, but there's a big velocity and movement separation between his pitches. Lefties struggle to figure out whether it's a changeup, curve or a 4 seamer and righties need to figure out if it's a 4 seamer, sinker or slider. If they all look the same coming out of the hand, it's just a guessing game. Guess wrong and it's a strike or mediocre contact. Because Bundy doesn't issue free passes, and he can get away with allowing more hits. WHIP is what's important and Bundy's WHIP is 1.21 (good enough).