You're going to have to support that claim a bit better. The core idea of RAID 1 is mirroring data, which BTRFS RAID 1 mode definitely does. Striping is not an essential part of RAID 1 (hence RAID 10), and reading data from two disks in parallel is an optional performance optimization that is not performed by all RAID 1 implementations (but could be implemented for BTRFS RAID 1: https://stackoverflow.com/questions/55408256/btrfs-raid-1-wh... ).
> Plus with mismatched drives, smaller drives go unused unless you write enough data.
Yes, the allocation is suboptimal from a performance perspective, as I've already said. But it is simple and straightforward and reasonably good at avoiding putting you into a situation where manually issuing a rebalance command is necessary. If you do need better performance, there's a RAID 10 mode. But since my NAS is currently a motley pile of SSDs, I don't need to to anything extra to have decent performance.