A lot of good info in this thread too.
For me the problem with staight in shots comes down to every liitle flaw that surfaces when we face them. They require "perfect" execution, related to every factor involved, from technical point of view to the position results wanted.
I'm not sure if they get harder because players are expected to make them, I know that since there is no large margin of execution error (I'm not talking about margins in OB-CB contact) this makes those kind of shots a good test for everyone.
From practical point of view and playing experience I would suggest that one may give some attention to the release of the stroke, watching the shaft following through in a straight line after contact. Ralph Souquet suggested that for the break shot in 8-9-10ball, it could be used for straight in shots as well.
Also, from the era of the old timers we already have the suggestion of not playing "too soft", in order to avoid slight deviations, as long as we play "center spin" anyway.
Nick Varner suggested to have a critical mind about what we expect from the shot in terms of position, it's better to play a stop shot on a long straight in shot rather than trying to draw full table length. Most of the times it's not really necessary and leaving a tougher position for the next ball provides better chances of run out compared to lowering the percentage of the straight in shot by trying to do "too much".
Finally, besides the well known practice test of shooting straight across table from the foot spot, having the CB returning to the tip of the cue, I would doubt the significance of any results outside competition. The real test is "on the field", and if you can't perform what is expected there then there is something that you need to work on, with a few exceptions that bring limits to the outcome for all players, for eg tough conditions. At the same time you need to remember that nobody is expected to perform 100%, take one shot at a time and move on.
Best of luck with your practice and progress.
Petros