I also accidentally click it all the time. It's a bit bonkers because it's right next to the collapse thread arrow. And if you're collapsing threads to mark which comments you've read already, you definitely don't want to accidentally click "next", because you'll have to scroll back up to accomplish your goal.
It would be a lot more useful if it stayed at the same position on the screen. It keeps moving to the left or right with every comment, depending on the length of the username of the person who posted the comment.<p>You could just keep clicking next without searching for it if it was at the same place above or beside a comment all the time.
I use it all the time on mobile, along with parent and prev. I do wish the touch targets were a bit larger but I would miss the minimalist design if they were a lot larger.