Cards like Sanguine Bond and Cliffhaven Vampire go right for the jugular without worrying about combat
For these decks, the opportunity of playing Jegantha, the Wellspring is just too high, not because of the number of cards they can't play, but because of the power-level of the small number of cards that don't meet Jegantha, the Wellspring companion restriction
You gain control of an opponents permanent when it enters and attack with it to remove a stun counter
Biorhythm is terrible in Limited too but they print plenty of bad Limited cards on bonus sheets, so its definitely possible
Dragonborn Champion Dragonborn Champion is tailormade for fitting into a deck with hard-hitting, evasive creature types
There are definitely combos possible here as long as the cauldron stays in play for its static effect and you can build your deck to take advantage of this card and have opponents just be a bonus