And the only "downside" of a9lh, if you call that a downside but basically only funnystory does, is that your 3DS can no longer boot without the SD card inserted.
Or if "can't be uninstalled without a pre-a9lh backup" is a downside too, then it makes two downsides, but who boots a 3DS without an SD card and who uninstalls a9lh ?