A comic can have a lot to take in when it has a lot of hidden details and Easter eggs that you need to look closely to notice. Also, if it combines different genres or styles, that can make it more challenging to grasp at first glance.
Well, there are several factors. Maybe it's the depth of the themes explored, like philosophical or social issues. Or it could be the richly detailed art that demands attention to every panel. Sometimes, a large cast of characters and their interwoven relationships can make it a lot to process.