The best example of the problem with Shonen
Naruto tells the story of a ninja named Naruto Uzumaki who wants to be the leader of his village. Naruto posses the typical personality of Shonen heroes, being rather dumb, childish and happy-go-lucky but ultimately good and kind-hearted. Even though this series has a very large cast of colorful characters, it seens that it works best when it focus only on a few of them because whenever it tries to involve a large ensemble problems with pacing start to show up. Due to the large amount of characters that need to be introduced, many chapters are made only of flashbacks or exposition that usually last a lot longer they needed to. This led the series to become infamous for the sheer amount of the so called filler, which is a lot worse in the anime series due to its format and having to keep up with the manga. Even though it had these type of problems from the beginning, the pre-time skip chapters are fairly entertaining and posses many memorable moments that justify the series success with the shonen demographic. The main problems with the series though, and by extension shonen in general, start to appear in the chapters post-time skip. After the time-skip the series goes through great lengths to introduce a large cast of new villains. This steals the focus away from all the characters we knew before and leaves many of them without nothing to do for a lot of chapters, only occasionally using said characters in filler or joke episodes. The magic the ninjas use in the series eventually becomes a source of problems with the writing as well due to he fact that it can basically do anything. This leads to several moments where the writer could simply excuse whatever new power a character had with it being a new technique, providing many Asspull moments. Many other inconsistencies like that eventually break the suspension of disbelief so hard that it becomes hard to feel invested in its universe again. With characters being nerfed left and right to make the villain of the week seen more threatening, boring or undeveloped characters stealing the spotlight even from the protagonist, a handful of villain-sues with weak motivations and stupid "take over the world" schemes and a very unsatisfying ending, the series quickly falls apart becoming nothing more than a low quality cliched shonen that should've ended many years before it was allowed to.