Why recommend ext4 over btrfs?

In
https://tracker.pureos.net/w/installation_guide/live_system_installation/
author Mladen Pejaković wrote

> Choose ext4 as filesystem, as it is the most stable and most tested.
> Don't use btrfs on your production builds!
No reason is given, no evidence is linked to back this view. Why is btrfs
so disrecommended?

I couldn't find a contact on https://tracker.pureos.net/p/mladen.pejakovic/
or a link to a mailing list on https://pureos.net/ but I thought PureOS
hackers might read a mailing list such as this.

Thanks.