ZFS lacks some features that btrfs has, such as creating CoW clones of individual files (rather than having to snapshot a whole subvolume).

personally i've been using btrfs on pretty much everything for about two years, ranging from multiple >100TB filesystems spanning 8 spinning rust drives to individual flash drives and had very few issues (compared to my experiences with ext4 on mdadm). snapshots/reflink copies have made many of my workflows much easier, adding/removing/replacing devices pretty much Just Work™, and the fact that everything is checksummed gives me a piece of mind i didn't know i needed. sure, ZFS has pretty much the same featureset, but it's not in the mainline kernel and seems to lack some of btrfs' flexibility (from the research i've done in the past, like adding/removing disks to an existing pool is still experimental).

what i'm really excited for is bcachefs, which takes what i consider the best features of both btrfs and ZFS and then steps them up a notch (e.g. ability to configure RAID settings and prefer specific drives on a per-file/per-directory level). as soon as it's stable enough to be mainlined i'll definitely be migrating most of my btrfs filesystems to that.