I recently tried it on Nexus 4 & it is certainly capable for daily use. It is running on 16.04. It will provide regular Ubuntu OS updates but the kernel remains the same as the android version.
Since the android kernel is dependent on the manufacturer, once the updates are stopped, the device is at higher risk even if it runs Ubuntu Touch, LineageOS or other 3rd party ROMS though they release regular updates.
I believe, postmarketOS aims to address this in long term by using mainline kernel & at present uses android kernel for drivers without the user space? I'm not sure whether this offers any particular advantage over UBPorts Ubuntu Touch at its present state, but I really appreciate what they are doing & looking forward to use/contribute to it soon.
[1]: https://ubports.com/
Which has finally driven Google to change their Play Services access contract.
https://postmarketos.org/blog/2018/06/09/one-year/#mainline-...
Android can’t be trusted anymore unless major change of policy. By extension AOSP are better but still dubious since they can’t totally cut the cord from Google if needed (as far ad I understood).
Without taking away anything from your argument, I'd like to point out that by most objective measures, Android is the mainstream Linux stack.