The only thing it's missing before I consider it full-featured is stable RAID 5/6. But it looks like that hasn't been forgotten.
My strategy to pull new things is to have one big feature that has ideally been reviewed and iterated in the mailinglist or there was a lot of testing already done. In addition two smaller features can be merged, with limited scope, not affecting default setup and possibly easy to debug/fix/revert if needed. Besides that there are cleanups or core updates going on so this should not touch the same code to make testing less painful. With new features the test matrix grows, code might need wide cleanups or generalizaionts before the actual feature code is merged. So this can indeed slow down development.
The raid56 is progressing but until the 6.2 pull from today there was not much to announce regarding stability/reliability. There were proposed fixes but as incompatible features, which means some changes on the user side and with backward compatibility issues. What's pending for 6.2 should fix one of the bad problems at least for raid5.
I've been running BTRFS RAID6 since 2016 and have only had one issue (arch kernel needed to be rolled back) and never suffered anything catastrophic. It's perfectly happy humming along with a 15x8TB raid array.
(It's perfectly safe to run metadata on raid1 and data on raid5/6.)
Although these drives have surpassed 50,000 hours of running time, I am fairly certain the day of reckoning is coming, but I have backups and a plan to recover.
I may recover to something not BTRFS. Not sure just yet.