Yeah like I was discussing with my friends about this it seems like her ult is meant to be synergized with a dps player's ult.
It appears the damage boost it gives is 100% (double damage) and the damage reduction it gives the recipient is 50%. If you were to tag that on say a Soldier's ult it could absolutely TEAR if the other team didn't have a Rein sitting around and it would instantly devastate a team if you were to give it to a Reaper when they ult.
You possibly also tag it on a Winston so they can jump in and sac for the enemy Mercy. With the damage amp + damage reduction combined with his large health pool it could be effective.
edit: Her damage buff from the ult only grants the recipient 50% damage.