In PES, if your team (or your player in the case of BAL) have high Tenacity, the opponent will be harder to play against than if your team (player) have low Tenacity.
This is why people have vastly different experiences with PES. So, if you edit the whole DB to have high Tenacity and play Regular difficulty you'll have a more difficult ML career than a person that uses regular DB at a harder difficulty setting.
If you want a long and prosperous (somewhat easy) BAL career, make sure that you start out with very low Tenacity. If you want an easy ML career, buy good players with low Tenacity.
Use this information to "tune" the database difficulty more to your liking. The ideal difficulty in my opinion is between 70-75 Tenacity in BAL. If you play ML the rough average for your team works as an estimate.
By using this as a "difficulty slider" rather than the actual deifficulty setting you'll get less "AI cheating". Regular difficulty has little of that by default and the higher the setting, the more the AI cheats and virtually makes the game worse. So, it's YOUR teams' or players' Tenacity stat that matters, not the opponents for some reason. Konami is a strange bunch for sure.
This information is relevant for all PES versions that uses Tenacity. The older versions probably work similarly with a similar ability which would be Mentality if I'm not mistaken.
PS. The adverse effect is that the higher the Tenacity, the faster the gameplay is as well. There is also a problem that at some point the AI becomes impossible to wrong-foot or get off balance so dribbling becomes more or less futile at some point which is around 85 average Tenacity for your team.
This is a very interesting finding! Now, what did konami think? I mean, If our team is better in mentality, the CPU plays harder?
In fact, I always thought PES worked like that. If you notice, when you get close to a star defender, your player suddenly has less ball control, or the reponse time reduces. We can see it in older versions, but its more frequent in newer versions. So, when playing against a super team, its not harder. Its just your team that gets dumber.
Matt10, maybe that explains why its been always easier to beat top teams and harder to play against others?