I don’t daily drive one but I’ve been keeping a close eye on it and here’s my opinions: postmarketos seems to have the most momentum out of any distro (you can see device support here). I do believe it will be viable eventually as a lot of work is actively being done. This month they announced two grants that were accepted for 4g volte calling and firefox on mobile improvements. They are also working on porting systemd to alpine so that gnome mobile and plasma mobile can be run without any workarounds. Also the oneplus6/6t seems to be the most hopeful for a daily driver.

tldr: I don’t think it’s currently viable but work and money is currently being put towards projects to fix that