Geeknamese
Explorer
I'm of the opinion that any time you give everyone a bonus that comes as a class ability, it cheats that class out of what makes them special. As a Rogue, knowing that everyone got advantage on surprise rounds would either make me not want to choose the Assassin archetype, or be disappointed if I had already chosen it.
That being said, if you like adding this rule to your game, don't let me stop you. There is an element of realism to it that Flatfooted in 3E captured, and I understand if some players miss that. I'd just suggest giving the Assassin archetype something to make up for that choice.
Trit
To gain surprise you need to be stealthed otherwise, everyone sees each other and initiative happens normally. If unity are stealthed to gain surprise, you have the benefit of the unseen attacker which is advantage on attack rolls. This means that if you have surprise, you have a full round of actions and advantage on all attacks. There is no way to gain surprise without the element of stealth which means the attacker that is stealth end will always have advantage.