I have Kennen, Heart of the Tempest legend not empowered and a hidden Switcheroo on battlefield. Do I empower Kennen if played from face down Switcheroo has been countered by Not So Fast or Hard Bargain?

No — if Switcheroo is actually countered, Heart of the Tempest never triggers, so your legend stays un-Empowered.

Abilities that trigger "when you play a card" only trigger once the play is completed by the card resolving; if a counter prevents that resolution, the trigger never fires at all (Rule 420.4.a.1, reinforced by 426.1.b). That holds for Not So Fast, and for Hard Bargain too — but Hard Bargain is conditional: if you pay the [2] on resolution, the spell isn't countered, Switcheroo resolves normally, and since it was played from facedown rather than from hand, the legend does get Empowered. If they paid Hard Bargain's [Repeat] [2], you'd need to pay [2] for each counter instance to keep it alive.

Either way, remember Switcheroo played from Hidden must choose both units at the battlefield where you hid it (Rule 811.1.d.2).