The primary issue here though isn't open source vs not, it's how updates and changes are rolled out.
What you're actually looking for is a phone where you manually initiate updates, and where it's guaranteed that change in behavior (e.g. theough experiments) can only be controlled through the device and not remotely.
With OS it's more likely to get that, but not guaranteed.
It won't happen, IMHO. That's because we have a whole pile of.. technologies, that are making good security nearly impossible. So if they want to avoid half of android users getting owned at the same time, they need to be able to push live updates.
Bugs are part of it too (smaller I think), everything is moving fast enough that early reporting mostly took place of extensive testing.
What you're actually looking for is a phone where you manually initiate updates, and where it's guaranteed that change in behavior (e.g. theough experiments) can only be controlled through the device and not remotely.
With OS it's more likely to get that, but not guaranteed.