In terms of abilities, beyond simply being a Knight himself, Oakenshield also lets you recur one creature a turn from your graveyard to your hand, giving you the staying power you need in a long Commander game
If something causes it to transform, it will do so if possible
Triggering off of artifacts as well is a huge deal since it lets you draw cards from Treasures, Clues, etc
But in formats where players can hold four copies of each card, legendary is still a massive drawback
This is what Commander uses to decide whether a card fits your commanders colours
The only black based decks which dont play Thoughtseize at all in Modern are highly synergistic decks like Goblins or Mill, or decks which cant afford to play it due to other restrictions like Living End