1 - increased complexity, makes troubleshooting harder

2 - higher risk of losing data

3 - harder to explain and understand vs ext4.

4 - has more special needs. in some cases you need way more free storage for rebalancing stuff around... you don't need that for ex4.

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now ext4 is surely worse than btrfs... but for the average home user btrfs is more of a problem than a solution, probably.