My conversion went fine, but there were so many misaligned sectors and constant strange checksum errors (on files written after the conversion). With the cherry on top being that if there’s more than X% of checksum errors, btrfs refuses to mount and you have to do multiple arcane incantations to get it to clear all its errors. Real fun if you need your laptop for a high priority problem to solve.
Lesson learned: despite whatever “hard” promises a conversion tool (and its creators) make, just backup, check the backup, then format and create your new filesystem.
I've never had the conversion corrupt a filesystem for me (plenty of segfaults halfway through, though). It's a neat trick for when you want to convert a filesystem that doesn't have much on it, but I wouldn't use it for anything critical. Better to format the drive and copy files back from a backup, and you probably want that anyway if you're planning on using filesystem features like snapshots.
Windows used to feature a similar tool to transition from FAT32 to NTFS. I'd have the same reservations about that tool, though. Apple also did something like this with an even weirder conversion step (source and target filesystem didn't have the same handling for case sensitivity!) and I've only read one or two articles about people losing data because of it. It can definitely be done safely, if given enough attention, but I don't think anyone cares enough to write a conversion tool with production grade quality.
Lesson learned: despite whatever “hard” promises a conversion tool (and its creators) make, just backup, check the backup, then format and create your new filesystem.