That alone makes them extremely powerful - starting a game with extra card in hand, and one that is guaranteed to work well with your strategy, gives an tremendous advantage over players who don't use one, and most of their deckbuilding restrictions were either easy to fulfill, or had enough loopholes to work around them
Everyones favorite Kalos starter is back to its bench-sniping shenanigans with Greninja ex
Once Sephiroth flips, which is easy to achieve with an Accursed Marauder or any kind of Grave Pact effect, the Blood Artist effect becomes permanent, and you get access to a terrifying 5/5 Flier that can draw you several cards a turn
Ready to see yourself as a real Pokmon Trainer
However, there are two small wrinkles to actually putting him at the helm of a Commander deck
Underrated EDH Card #32: Necrologia ($1.15) Necrologia is super mana-efficient card draw