There are good short balls and bad short balls, but sometimes their goodness or badness is an entirely post-facto construct. Take two balls
Pat Cummins bowled, back-to-back, to Pathum Nissanka in Lucknow. Both were banged into roughly the same area of the pitch, and both climbed to just over shoulder height and finished outside off stump.
Nissanka pulled the first one for four, picking the length in a flash and dispatching the ball well in front of square.
The line of the second short ball may have been ever so slightly closer to Nissanka's body, cramping him ever so slightly for room, or Nissanka may have taken ever so slightly longer to get into position for the pull. In any case, he failed to get on top of the bounce, and hit the ball in the air, within range of David Warner haring to his left from deep square-leg.
Similar balls, different outcomes, and in each case there was only so much the bowler was in control of. Bowling fast is an intensely physical act, bowling fast and short even more so, and how quickly the ball reaches the batter and at what height and line are hugely dependent on the vagaries of the pitch and how the ball reacts off it.
Bowling short is, in essence, an act of faith.
The variance of outcome between those two short balls from Cummins is also typical of that length. It's a length that's likely to go for runs, but it's also likelier than most other lengths to bring wickets.