When you already have too many channels, and then they give you the ability to have too many threads.
I feel like they do a worse job at the supposed value add of being able to archive and later refer to a topic of conversation than channels simply because they are so light weight and proliferate so many of them, potentially.
Plus it's really jarring to be pulled into a bunch of threads.
Slack segregates by interests and social group, and bundles that with strict gatekeeping. There is no way for people to remix that to suit their own preferences. You can only fragment existing communities more.
I would love a slack multiclient where I can put channels I care about side by side, regardless of origin, and keep the rest out of sight. Instead now everyone has their own #random and #offtopic and so on. Cruising between Slacks and Discords is like navigating a hall of mirrors. If there is disagreement, the only solution is complete schism.