The Onslaught perk gives a 20% boost to your auto-attack damage, spell damage, shields, and movement speed, but trades your ability to get any defensive, mobility, or cloaking masteries. By choosing this perk, you are committing to your damage and giving up any type of survival based mastery. This is best chosen for those who do not die easily, who need the cloaking, or who are good at avoiding bad situations/damage overall. The heroes that benefit the most from this mastery is Tychus, due to his massive amount of armor from talents, making his defensive masteries rather useless. Anvil, if the player behind the anvil knows how to properly position themselves in the lategame when there's no cloaking avaliable and finally Tassadar, if the player knows how to properly use force fields and stay out of danger. While it might seem nice on Nova, the cloaking offers too much potential to give up, and its pretty much useless on the predator and swann, because it removes the preds defensive masteries like block, and swann doesn't have anything that benefit from it.