I think that the only way you could speak of an underachivement would be if you take the last Heynckes team as measure stick. There is no question for me that the Bayern of 2013/2014 was interferior to the one one year before.
The Bayern of 2012/2013 was one of the strongest club teams in history and the most balanced and versatile side I have ever seen live. This side could switch up between a highly efficient possession style and a deadly counter system in the middle of a game and their well tuned pressing system gave you no time to recover. Their defensive stability made it seem that you basically have no chance to score they also did not simply defeat teams, they destroyed them without mercy or shifting gears. In short it was a team that seemed nearly unplayable when they were on top of their game, which was the case 95% of the time. The only time they looked threatened or on the back foot in top form was the first 30 minutes of the CL final, when Klopp´s Dortmund basically threw everything at them, which would cost the latter later in the game because they went out of steam.
There was one big downside of their way to play, though, which makes comparisions to other sides pretty hard. It was an extremely demanding system for the players, both physically and mentally. It did work the way it did because there was an enourmous motivaton in the squad. It was a team with a clear mission, who simply wanted it more than anyone else.
I believe that it is incredibly hard, if not impossible to uphold such an amount of motivation over a long period of time. That is also why the transition under Guardiola was needed to an extend. It did not work perfectly in all areas, but overall while it is less exciting and dominant and relies more on individual class in the offense to break deep lying defenses (a direct result of his system), it is more substainable.
In terms of expectations a CL win should never be taken as measure stick of a successful season. It is the hardest competition in the world, where one slip up can end the campaign. What can be expected is the Group win and thus a normally doable draw until the Semi Finals.
The German cup is similar to an extend, maybe even more volatile given the one leg nature but with far fewer true threats to them. Either the win or a close loss in the latest stages should be expected.
In the end the fairest and best way to evaluate a season performance is still the league. In Bayern´s case the expectation is simple: everything less than the title is a failure. If you are in such a commanding position in terms of ressources that you basically pay your players as much as the two biggest rivals put together, then you simply have to win a battle of consistency.
We weren't inferior from a tactical point of view. Pep changed things, but they worked. I remember fairly well, how we played in Heynckes´s first year. Not bad, but also not fantastic and dortmund won the league. At this point nobody talked about heynckes as tactical genius, who is going to form one of the best teams of all time. The general consensus was that he is a solid/decent/good manager, who can manage the youngsters and stars, but who is lacking tactical finesse. Just one year later and he is a tactical mastermind? I dont think so. Glorification of the past at its best.
In december 2013 there was almost no expert/pundit left, who thought, that bayern declined.
The difference between 12/13 and the year before and after was, that bayern "was a team on a mission". That had nothing to do with tactics. Everyone was in top form and everyone played sole for the one goal to win the CL. At one point they started to believe that they are unstoppable. We still have a good team spirit, but its impossible to recreate this very special feeling in and around the team.
Pep made two decision, that deserves criticism.
1) Why did he change so much after the winterbreak? It hurt the team to change things up on a weekly basis
2) After losing the first leg against Real, he panicked and changed things to please other people. Pep gave up his philosophy, which resulted in a very bad lineup.
I doubt that he´ll do this mistakes again. Other things that heavily influenced our bad finish were just not his fault. There are lots of factors, that are random/hard to influence for a manager. In 12/13 all these things were going in our way, while in 13/14 a decent amount of things didnt (e.g. injuries; ribery out of form; winning the league way to early). As Dortmund fan you should know these things fairly well.
so what the conclusion? Criticism, that is primarily based on results, doesnt make any sense; especially in cup competitions. We played a very good season and Pep did a good job. 99% of the people who suggest otherwise are completely deluded.