Did a little search...I'm not an analytics guy (I'm OLD), but I found numbers to back up what I've been seeing.
Weighted On -Base-Average Allowed by Pitch Type:
Four seam FB: .350
Sinker: .349
Cutter: .315
Change up: .292
Slider: .269
Curveball: .263
Splitter: .257
Hitting approaches always focus on Fastball-First...They sit on it as much as possible, and if you are in the big leagues, you can hit the fastball, even 100+.
In looking at pitcher spray charts, for a good/great performance, the fastballs are grouped in specific areas:
Four-seam FB: Outside edge, and top of the zone. Miss in the zone, it gets hit, often hard
Two seamer/Sinker: Bottom of the zone, down and in for same-side, swing-back and on-to-off the plate for opposite side, depending on shape.
The life on the 4-seamer has changed it a lot...Castillo's 4 seamer runs arm side hard, his 2 seamer sinks.
I agree that Brash and Munoz need to tighten up the FB command, a lot like Gilbert / Kirby...when they can dot the outside corner, they are really tough to square up. The out pitch is the slider, but they gotta paint with the FB to get there.
Your comment about bad hitters reminds me of what my dad told me when I was about 12...The Three Nevers: Never play cards with a man named Doc, never eat at a place called Mom's, and never throw a change up to shitty hitter.