I often think that comics are the most difficult medium to work in. With movies, you have the visual element, as you do in comics, but you also have music. Music is one of the most powerful things out there and it can easily incite emotion, set up atmosphere, and cause “the feels”. Even the visuals in movies are highly sophisticated because they are essentially real life in that sense.
With books, you have the room to create worlds, and cause ties and emotions with words. But comics are at a considerable disadvantage here too. They just can't accommodate as much text.
Comics don't have musical accompaniment (usually, unless it is a web comic and even then, accommodating music to the page would be a tricky thing because everyone reads comics at different paces). They have visuals, obviously, but creating a sense of motion is considerably more difficult than in films. And the visuals are at the mercy of the artist's skills. Comics also don't have the luxury of word space, as novels do. There are less words, so it is more difficult to capture the audience's attention.
A comic has to be a perfect blend of good, concise writing, and engaging visuals, along with an interesting plot and relatable characters. While films and books require similar things, the stakes are higher in comics. There is a shorter time frame to capture the readers, less story that new readers are willing to go through before they decide if the comic is worthy of their time.
Another problem that comics face, especially ones that are more action oriented, is the pacing. Comics take an incredibly long time to produce. Sure, novels and movies take a while to create as well, but both of these deliver the story all together. With comics, the story is usually presented little by little (this is both if the creator has a one page a week posting schedule or say, a chapter a month schedule). It is very rare that an entire webcomic is created completely and then all the pages are posted. At the very least in such a scenario, the pages would be posted a page a day, which still cuts down on the instantaneous-ness that can be presented with a book or a movie.
An action scene might take out two months to play out if there were, say, eight pages involved in the sequence. That is a really long time to keep up the momentum of the action.
Earlier I mentioned music as an easy means of inciting emotion in the audience. This is a great challenge for comics. With webcomics, it is theoretically possible to add music to the pages, but the addition would not be as flawless as, say, a movie score is to the film itself. It just seems that it is extremely hard to create an emotional connection with the reader through the comic medium. I would argue that this involves a lot of artistic and writing skill to move people.
And yet despite all of these disadvantages, comics still persist. What is the appeal? For me, when comics are done right, they become a perfect blend of visual art and good writing. I love running across visually stunning webcomics and then sticking with them because I have come to care about the characters and what happens to them.
It is a medium with the potential to be story telling at its best.