Call-up pitchers that can execute the game plan called by the team are as good as an experienced MLB pitcher. That's not trivial because the margin for error is much smaller against MLB hitters. Most hitters struggle adjusting to MLB movement pitches because they are thrown with better control for quality strikes and/or later movement that triggers swings on pitches well outside the strike zone.Not all those hitters have those jitters. Which is why I said "many."
And, assuming they don't, they get the book on the pitcher faster (i.e., before the first at bats) than the pitcher's team can get an accurate book on the hitter. On the pro level, with today's sophisticated scouting systems and speed of information transfer the book on the call up gets there a lot quicker than it used to. Even if each have the "book" on the other, both parties still have to execute with one inevitably failing each and every time.
For both, the first call-ups are eye-opening. The smart ones learn from the experience and go to work on developing their skills to be successful at the MLB level instead of what works at AAA.