For me they create choice fatigue. I know it's no one's fault, but sifting through 100s of links to see which ones I might like on top of the stuff I'm already reading can get exhausting. I can see why some people hide promotion threads
Other issues with those threads is that it's a 1st come 1st serve basis since the top comments are the most seen.
Most of them are vague so people just drop their links and go. Ones based on specific genres, art style, settings etc, or specific rules requiring users to support each other tend to be a little less bloated
The biggest issue is that there are no readers here. If someone wants a new web comic to read they are either going to check the front page or ask for recommendations elsewhere like Reddit. I don't think the average Tapas/Webtoon reader knows this place exists