A cross seam ball is an honest ball

Presenting the seam sideways gives up control of the movement and gains something a batter cannot read at all.

Fast bowlers are taught from the first day to present the seam upright, wrist behind the ball, fingers either side. Everything about seam movement flows from that. The ball lands on the seam, the raised stitching grips or skids depending on the surface, and it deviates in a direction the bowler has some claim to be choosing.

The cross seam delivery throws that away deliberately and I think it is used far too little in the middle overs.

Held across, the seam is horizontal at release, so the ball can land on any part of it. Sometimes it strikes leather and skids on low. Sometimes it catches the seam edge and jags. The bowler does not know which. That is the entire point, because the batter does not know either, and against a batter who is set, uncertainty is worth more than control.

There is a second effect that matters more on a dry surface. A cross seam ball roughs up differently, and on abrasive grounds it is how you accelerate the wear on one side that you will later want for reverse. Bowlers who bang it in cross seam for a spell are doing housekeeping for the twenty fifth over as much as attacking in the twelfth.

The reason it stays a novelty is that it looks bad when it fails. A conventional ball that pitches on a length and does nothing looks like a good ball that beat nobody. A cross seam ball that sits up and gets pulled looks like a mistake, and bowlers are judged in the moment by people who will call it a poor delivery. So it appears once an over from a bowler with a captain who trusts him, and never from a young quick trying to keep his place.

I would use it in blocks. Four in a row, at a hard length, with the field set for the top edge rather than the drive. Consecutive deliveries are what make the variation work, since a batter who has seen three balls behave differently from an identical action stops committing forward, and a batter who is not committing is a batter you can beat with a normal ball afterwards.

The best thing about the cross seam is that nobody can tell you what it will do. Including you.