Does Ember Monk get the buff from playing a hidden card even if it is countered?

No, Ember Monk does not get the +2 might buff if the card played from hidden is countered.

In Riftbound, a card is not considered "played" until it successfully resolves. According to Rule 412.1.b: "A card that is Countered is not considered to have been played."

Because the trigger condition for Ember Monk is "When you play a card from [Hidden]," and a countered card is never considered played, the ability does not trigger.

Official Ruling:

"No, Ember Monk does not get +2. The hidden spell card must resolve to be considered played for Ember Monk's ability. If a spell is countered, it's considered never to have happened and does not count as being played."