Admittedly, I don't fully know how the catch up and gating works but seeing the other team just unleash super after 1 or 2 waves of envoys is nutty.
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The health gating is to allow for a comeback. Killing enemy guardians while invading will regen boss health. Killing waves of primevals will give an increasing damage buff against the boss. This buff after a number of waves will let you just melt the boss and win.