Long straight in shots are a great way to measure the baseline margin of error for your technique. There's no judgment involved in how much of a cut to make because the CB and OB centers are perfectly lined up. It's purely about getting the CB from point A to point B as accurately as possible. In a cut shot, you have to use judgment to decide where point B is (i.e. how much cut to make), but then it's just a straight shot to there with the same margin of error to point B as your regular straight in.
In fact, the more the cut for given CB/OB and OB/pocket distances, the smaller the total error for the shot geometrically. If you have a 4 degree error on a full ball straight in, that's a fundamental 1/16th ball error in your technique. At 1/4 ball hit that 1/16-ball error becomes 5-6 degrees, and at 1/8 ball hit is 7-9 degrees (more than a diamond across the table length.)
By introducing the extra judgment factor, it makes it more difficult to really know why you're making or missing shots. Did you miss because you picked the right point and your stroke was off, or did you pick the wrong point and hit it perfectly? Did you make it because you picked and hit the right point perfectly or picked the wrong point and hit it bad? Without a reliable baseline, iit's impossible to know for certain.
I think it's a common misperception players have when we say we make every cut but miss straight shots. I think it's due to statistics, and humans are wired to have bad intuitions about them (bad for us, good for casino owners.) We're obviously missing cuts too, or we'd happily be giving weight to Efren. The question is cut percentage vs straight percentage, and its not so easy to compare them properly.
In pool, we play for angles in order to have better position options, so we're already shooting many more cuts than straight ins to skew the numbers. Although there are exceptions, we're usually only straight in when we're out of line. That means we're likely shooting a harder than normal shot to begin with to get back in line, which will distort our perceptions of them even further.
These are some of the reasons to be careful about exaggerating the difference between straight ins and other shots. In a way, every shot is a straight in shot. Learn to embrace them for the fundamentally simpler shots that they are.
To practice them, use a long shot where the CB is 6 or 7 diamonds from a corner pocket and the OB is midway between them to minimize margin of error. Shoot a stun shot and try to stop the CB without any sidespin whatsoever. Sometimes you will make the shot but leave the CB spinning, which is bad. That means you hit the OB off-center and threw it in with english. Watching for a flat CB is super important when practicing a straight stroke this way (measles ball is helpful for this.) Once you can do the stun, then try following the ball into the pocket (moderately harder) or draw back into an opposite corner pocket (a *lot* harder.)
Good luck!
Robert