My personal take on all this is, promotions are too fast tracked, if you have a lot of good or great patrols. Why is this an issue, well.. let's examine for such being the case.
First & foremost, the game replaces units faster than the participants in r/l ever could or did. This is 1 serious downside to the whole immersion factor of the game.
Should you be fortunate, to actually make it from start to finish of the war, the total tonnage you'd end up with.. would in all probability, majorly surpass, if NOT out strip the actual tonnage, much less ships that Japan was able to put to sea.
As I understand it, with regards to promotion & the key elements that fuel it/drive it or much less govern the promotional chain, is tied into successful patrols & gross tonnage sank. If I am wrong on this, then by all means point out where I am wrong on that.. I won't be offended in the slightest. As I stated, this is what My out take is on the promotion process in game.
1 thing that I can recall with clarity, is that with SH3, thanks to JScones of SH3Commander, this tool working with the files of it resolved the issues with career length, in that it simulated career length (as realistically as was possible all things considered...). The only chief issue I have is that there was never 1 done up for SH4, or much less SH5 as well... ahh well...