Is the backlash just bc of this new character? Or also that the pacing is slow? Those are the main complaints I've been seeing. For me, I'm just annoyed with the use of last chapter previews at the start of each new chapter (that's more of a subjective complaint tho).
Story-wise, it's doing a good job in building a plot that's going to involve a LOT of characters, esp considering that it's using a foundation with a large cast and a lot of vaguely connected side-plots from the last 2 seasons. All that is going to be needed to come together somehow and/or be resolved, and you'll rarely find a good story that can do that quickly.
So, while the pacing is slow, it's filling out the hanging plotlines left behind from the last season just fine and giving new situations and developments time to breath, which I appreciate. But as the chapters don't seem to be episodic, it might be a better story to binge read rather than read each chapter as they're released.
As for the new character, I'd be upset if the story focused solely around him or if he overtakes the MC in regards to the importance of the plot, but the character seems more like a comic relief, in order to keep the tone from being too dour, and an audience surrogate, to let the readers have time to get to learn the changes and development happening with Shioon since the last season (and considering our protagonist seems to be barely hanging in there and is suffering from all the trauma from the last seasons, as well as having these heavy responsibilities now, just focusing on him might make the tone of the story be too depressing and serious).
I'll admit that it may walk a thin line at times, but it's clear that the story's main focus and concern is on Shioon and hopefully, as he recovers from his own issues and we see more of the story unveil, the focus will keep on Shioon and the main stuff going on in the Murim. Personally, I anticipate that the story will use the parallels of this new character in order to shake up the MC or the readers; it all depends on how the author uses him, but there is still good potential with this character to make an impact if the character is used right.
Ngl, this has the makings to being a long (and potentially drawn-out) series. I recommend waiting for more chapters (maybe 50) to come out before starting. At nearly 30 chapters in, I only now feel like the plot regarding the series' main conflict is beginning to move.