I felt like a fairly decent illustrator when I started my comic, and I can say now that making a comic is the quickest way to feel like crap about your craft, haha! Comics quickly show you every spot where you're lacking as an artist. One of the toughest challenges for me is figuring out how to move the camera around in an action scene without it being disorienting to the reader, and still maintaining the energy and dynamism of the camera. Pacing is another thing. In film, you can show many different things happening with very quick cuts, and you have real motion in the frame. In comics, that's much more difficult, so framing shots so you can see multiple things happening at once in an action scene, while communicating the movement, while still keeping an interesting camera angle can be tricky. Also, now you're forced to draw something from an angle that you're not used to, which is an added layer of difficulty. You can't fudge it, because it's necessary to the page layout and readability of the comic.
And of course just the tasks of designing so much stuff, drawing so many things you never have had to draw before, doing it all quickly and sticking to a schedule, these all can be really difficult. Also, working for a month and a half concepting and drawing pencils, without posting anything is rough. I desperately want to share what I'm working on but can't, so it's hard to keep my motivation up at times.
It all forces you to get so much better though! I'm only a couple issues into my comic, and already I can see a massive improvement in my art. If I had started with the issue I'm working on right now, it would have taken me twice as long and looked far worse.
tldr, comics are so haaaarrrrrdddd, but worth the effort!