I think this is worth considering. Like, sure, there are some stories that really won't fit in a specific medium, but for a lot of stories I think they could work for several different media, they'd just need to have different focuses. If Runewriters were a book instead, it'd probably focus in on some moments that are glossed over in the comic, and present them in different ways. It would highlight a different aspect of the story.
So part of the consideration isn't just "what's the ideal medium for this story" but also "what's doable for me?" or "what kind of story will I enjoy telling?" are important questions too. I like drawing and writing dialogue better than writing prose. So it wouldn't matter if a book were my story's perfect medium -- I would still be making a comic, and adjusting that story to play to a comic's strengths.
I would have to think about this more specifically, to be honest! What makes a general story ideal for webcomic form? Uh.... visuals, I guess??? The story works with the pacing necessary for an update schedule????
But if you ask me, "what are the strengths and weaknesses of the medium for YOUR story," that's easy! A story full of sign language and charades is an odd choice for a comic -- those things take up a lot more room and have to be sort of approximated in order to feel natural. BUT, different forms of communication can be presented visually, in a way that just wouldn't work in a form that has audio, where everything that needed subtitles would become a little more alienating.
So, looking at it like that! I could also examine what parts of my specific story would be strengthened by prose, and which parts I would hate to lose. Same with an interactive experience -- how would the story need to be reworked to fit it, and would any part of the story be much more powerful if presented that way?
I think it's more useful to consider these things specific to your story, rather than creating a List Of Webcomic Strengths and then holding your story up next to them to see if it has any of them, because sometimes the strength of your story as a comic/book/game/movie/rock opera/podcast is gonna be something that's unique to your story.