Well, it could be that some people find the content too explicit or lacking in depth. Maybe the storylines are weak or the characters aren't well-developed.
I think the reason could be that the plotlines in some science fiction works are confusing or lack depth. Also, the special effects might not always live up to expectations.
The Marmaduke comic could suck due to a variety of reasons. Maybe the humor is outdated, the art style isn't appealing, or the storylines are too repetitive.
Well, fan fiction might suck for various reasons. Maybe the characters are out of character, or the story just doesn't have a coherent structure. Also, some writers might lack the skills to make it engaging.