And since you should probably be running a modern CoW filesystem then you probably shouldn't be using swapfiles ... is the conclusion I arrived at anyway. Either install multiple times more RAM than you need and enable zswap or else set aside a swap partition as has always been done.
Swapfiles on BTRFS seem to work great if you allocate them early (with the command linked in your article) and put them in a different subvolume than the rest of your system.
BTRFS in general seems more I/O constrained than ext4 or xfs so it's probably still worse to put a swapfile on a BTRFS partition, but so far my systems are doing fine.
> BTRFS in general seems more I/O constrained than ext4 or xfs so it's probably still worse to put a swapfile on a BTRFS partition,
AFAIK, on Linux swap bypasses the filesystem. It asks the filesystem for the extents which are used by the swapfile, and does the I/O directly on the block device. This explains the many restrictions swapfiles have on btrfs (the file can't be copy-on-write, can't be mirrored, etc), but it also means the speed for a swapfile will be the same on btrfs as on ext4 or xfs (assuming a similar layout, that is, the file is not too fragmented; IIRC, the kernel prints the number of extents when you do a swapon, so you can see when it's too fragmented).
https://btrfs.readthedocs.io/en/latest/Swapfile.html