I'm certainly not underestimating the success of Bayer Leverkusen. 51 game unbeaten streak and undefeated championship title are not easy achievements. But if we consider that their toughest and most serious opponent this season is Bayern Munich and Bayern's poor form this season, Leverkusen's success does not seem like a dream achievement. Because Bayern Munich were really bad this season. They even finished third in the league behind Stuttgart.
Would you say Bayern was bad last season when they won the Bundesliga? I'm guessing your answer would probably be no, but the fact is, this season they've scored 1 point more than in the last one. They also got through to the Champion's League semi-final and fought a good fight against the winner, Real Madrid.
They fail to achieve their goal in the league, but calling that a "really bad" season is a bit ignorant.
Bayer Leverkusen has won the league undefeated, established a new record of games without a loss, and were very close to setting a new record of most points in the league.
There's just no pleasing you, it seems.