The stability issues with RAID5 on BTRFS tend to be blown out of proportion. I've been using it for my home server with mixed drives of ~50TB raw for about 5 years without any issues. I use it for the same benefits you've noted, mainly support for ad hoc expansion while still maximizing usable space. Until bcachefs is released BTRFS is the only filesystem I've seen with these features.
That said, the mentioned stability issues, particularly the write hole, ARE real and possible. I wouldn't use it in a commercial production scenario or for data I couldn't stand to lose. Anything I care enough about has off-site backups, and my server runs on a UPS to mitigate issues from power outages. But for my purposes it's worth the minor risk for the benefits.
When 99% of traffic has already formed a single lane then the guy trying to go past a bunch of people and cause an extra unnecessary merge is the asshole and I'm not letting him by.