Some features such as Raid5 were still firmly in "don't use if you value your data" territory last I looked. So it is important to be informed as to what can be used and what might be more dangerous with btrfs
What makes you say that? I've seen plenty of people make this claim based on URE rates, but I've also not seen any evidence that it is a real problem for a 3-4 drive setup. Modern drives are specced at 1 URE per 10^15 bits read (or better), so less than 1 URE in 125 TB read. Even if a rebuild did fail, you could just start over from a backup. Sure, if the array is mission critical and you have the money, use something with more redundancy, but I don't think RAID5 is infeasible in general.
* All localized error are correctable, unless they overlap on different disks, or result in drive ejection. This precisely fixes UREs of non-raid drives.
* If a complete drive fails, then you have a chance of losing some data from the UREs / localized errors. This is approximately the same as if you used no RAID.
As for URE incidence rate - people use multi-TB drives without RAID, yet data loss does not seem prevalent. I'd say it depends .. a lot.
If you use a crappy RAID5, that ejects a drive on a drive partial/transient/read failure, then yes, it's bad, even worse than no RAID.
That being said, I have no idea whether a good RAID5 implementation is available, one that is well interfaced or integrated into filesystem.
It has terrible performance problems under many typical usage scenarios. This is a direct consequence in the choice of core on-disc data structures. There's no workaround without a complete redesign.
It can become unbalanced and cease functioning entirely. Some workloads can trigger this in a matter of hours. Unheard of for any other filesystem.
It suffers from critical dataloss bugs in setups other than RAID5. They have solved a number of these, but when reliability is its key selling point many of us have concerns that there is still a high chance that many still exist, particularly in poorly-exercised codepaths which are run in rare circumstances such as when critical faults occur.
And that's only getting started...