A significant advantage. There is no reason to summon your Primeval first. In fact, it’s MUCH more often than not worse to do so. The team that summons first gets significantly less invasions and has to deal with the blockers. With Taken goblin blockers shielding the Primeval and envoys. If a team summons theirs when the other team has less than 30 motes, it should be a shoe-in. Chances are though that it will actually cost you the match. You’ll spend the majority of the time just having your Primeval healed until the other team summons theirs and then it literally only takes a single enemy invasion where they get a couple of kills and you’ve already lost. This is especially worse on Emerald Coast because that map is WAY too small. Gambit is broken. Prime is better but still broken because of this. I’ve said this since the original Gambit first dropped. BOTH team’s Primevals should arrive at the same time and the team responsible for summoning them should get an advantage for doing so. That’s how it needs to be arranged. Trying to rank up once you hit Legend is excruciating because of how broken this is.
Typically the team that summons first wins because you don't start building stacks until you summon it. Sure they can invade that's part of Gambit if or when they summon theirs you can invade back. Also don't be a dork and waste your super on them right away unless you're a tether hunter. Build up some stacks wait for the invasion then burn the boss after you kill the invader. It's a pretty easy concept.