1. Its maintenance/activity appear sporadic at best. 2. It's inscrutable. Whenever I've had issues, I don't have any idea even how to approach solving them. Half the time they reference in-depth mathematical terms that just are not relevant to anyone who isn't implementing the language 95% of the time. 3. The tooling often just... doesn't work. I'll go to run a commend given in the official tutorial, and it fails in a way that is just beyond me, as an experienced engineer.
TBH, I'd really love to use it. I've been finding myself using `fp-ts` on many of my projects for just such a reason, and have even set up `unplugin-auto-import` to automatically detect and import those modules, so it's like I have more in my stdlib. I've even thought of making a true language out of these primitives.