Nextcloud, and before it Owncloud, have been "in production" in my household for nearly a decade at this point. There have been some botched updates and sync problems over the years, but it's been by far the most reliable app I've hosted.

In terms of privacy & security, like everything it comes down to risk model and the trade-offs you make to exist in the modern world. Nextcloud is for sharing files, if nothing short of perfect E2EE is tolerable it's probably not the solution for you, not to mention the other 99.999% of services out there.

I think most of the problems people report come down to really bad defaults that let it run like shit on very low-spec boxes that shouldn't be supported (ie raspi gen 1/2 back in the day). Installing redis and configuring php-fpm correctly fixes like 90% of the problems, other than the bloated Javascript as mentioned in the op.

End of the day, it's fine. Not perfect, not ideal, but fine.