It's proven, battle-tested, pretty robust and you don't rely on any specific vendor formats or any hardware for that matter. The main point would be to keep it simple. You could use BTRFS or ZFS or all kinds of things. But it only introduces additional complexity and points of failure. And has no benefits over a plain mirror (what the RAID1 does) if we're talking about just 2 devices. At least it served me well in the past. Contrary to cheap hardware RAID controllers and also BTRFS which also let me down once. But a lot of development went in to that since then and the situation might have changed. But mdraid is reliable anyways.