I'm sure it depends on your use case etc., but in my work context I find they function really well.
I did try test some concepts around Slack/Discord threads serving as entire "discussions" for a literary community (so you e.g. post a topic and discuss it in the thread under the topic), and in the end there were a bunch of small pain points that all added up and made it unfeasible.
At work though I find Slack threads to be a good middle ground for having discussions in a medium-large channel with interested parties - everyone can see the main channel message, and those who choose to engage further can post in the thread or opt to receive thread updates if they just want to "listen in"... then if some important conclusion is reached you can opt to send that thread message out to the main channel as well to catch other people up on what was discussed.