Why it sucks:
Ugly and laughable character designs, especially for the two titular characters, who look like superhero rejects.
It overuses a lot of toilet humor, especially in the episodes "Pick a Nose" and "Fanboy Stinks". Examples:
Fanboy sneezing into his surgical mask when he is disguised as a nurse.
Kyle attempting to pull Fanboy's finger and Fanboy farting on him when he refuses.
Fanboy and Chum Chum hiding in a waste container.
The use of pop culture references, especially to R rated movies, are used in the weirdest way imaginable.
The names of Fanboy and Chum Chum are just silly.
The annoying voice acting, even from actors such as David Hornsby, Nika Futterman, Jamie Kennedy, and Jeff Bennett. It also mostly consists of screaming and squealing.
The titular characters are annoying and unlikeable protagonists who always hurt Kyle and Lenny every time they encounter them, and worse still, they never get punished for their actions, they also often do not realize they are bothering Kyle and Lenny, but at least they improved themselves in the second season.
On that topic, some other characters can also be unlikable, (such as Lunch Lady Cram/Bus Lady Cram) except Kyle.
Kyle's cliché of being treated like dirt is insulting, turning him into a Butt-Monkey, a character who gets hurt by everyone in the show for little to no (if not any) apparent reason at all!
Bad pacing that feels like the writers weren't even trying.
Some characters are clearly using the same models, but with slightly different appearances, different names, and classifications. For example, Lunch Lady Cram's twin sister, Bus Lady Cram, is basically her but as a bus driver. She has the same appearance as Lunch Lady Cram (the only difference is the hat), the same personality of Lunch Lady Cram (only much more hurtful, cruel, and sadistic), and even the voice actor of Lunch Lady Cram, Candi Milo (only her voice is slightly deeper and has a much more evil laugh). In one episode, before her name was revealed, everyone in the show even called her Lunch Lady Cram because of this.
The background music sounds distorted and broken like the creators were lazy and couldn't even check for sound.
The theme song is annoying and laughable, half of it consists of "bah bah bah"s. Keep in mind that the composer of the theme song also did the Gravity Falls Intro.
The majority of episodes are painfully nonsensical & don't have any logical premise, as if they were crafted by hyperactive minors.
Various animation errors.
There is so much noise that never stops nor ends.
The episode "Moppy Dearest" has Object sexuality
jokes in a kids' show.