Obviously stealing planeswalkers can be very powerful, especially if you can ultimate them right away, but Callidus Assassins flash, as well as the fact that it destroys the creature rather than having your opponent get it back when it leaves the battlefield, are significant as well
Some of them offer specific fixing (or general fixing, like Birds of Paradise)
Midrange - Much like blink decks, Midrange builds regularly lean into the card advantage of longer-term things like creatures with ETB abilities
Recently, the Commander Legends set added over 70 legends to the format, some new and others reprints
However, a Planeswalker essentially acts as another player, so when an opponent attacks you, he or she must specify if they are attacking you or your summoned Planeswalker
There are a lot of really busted artifacts in Vintage Cube, and it makes sense to include answers to them