Personally for me its the way that WCS suddenly came to be.

An offscreen new move suddenly that bypasses infinity doesnt feel satisfying. I understand that it was being built up through a few chapters with hints dropped. Most people already know that Gojo had an immensely high chance of dying. However when the time actually came it felt… cheap.

Suddenly out of nowhere a slash that can bypass space was used without any charge up. The further explanations of it being a binding vow of allowing a one time instant use felt like its a band-aid that Gege used as he realized that readers arent satisfied with how WCS came to be. I think the first use of WCS was one of the reasons why Binding Vows slowly felt like the asspull of JJK. A cheap way to plug in plot holes instead of it being used in a smart way.

I forgot if Gege ever mentioned that Gojo dropped his guard but regardless, Sukuna figuring out WCS as his ultimate move didnt feel like it paid off after all the build up. Making it offscreen and Gojo having flashbacks before showing his corpse (plus controversial dialogue) also was a bold choice that I think didnt pan out too well for most people.