Calamares sets up subvolumes for root and /home by default, although there are still some problems.
In my case; I have two separate btrfs partitions, root and /home, and I DID NOT want subvolumes, however the installer set them up anyway, even if it doesn’t make any sense. Now I have a SV named @ on root, and one named @/ on home. It doesn’t hurt, but it’s not perfect either…
Still, everything is working as it should. I see no reason to not use btrfs with Manjaro.
I don’t use compression in order to keep things simple.
As for COW, sometimes you need to turn it off, like when dealing with virtual machine files (.vdi in VirtualBox), else performance would suffer. This is detailed in the Arch wiki.
To get back to your original question: you can easily use btrfs with Manjaro without a problem. The installation procedure is not perfect, but it gives you a fully working installation with btrfs. And then there’s always the manjaro-architect installer which offers more fine grained control over partitions and mount options.
(from a happy btrfs user since many years)