Should gamers be able to get a refund if a game is broken at launch such as Unity and Halo: MCC?

[Augen]

At the discretion of the publisher really. If anything gamers should pre-order less when a given publisher has lost trust that games were properly tested. Assassin's Creed III was such an unplayable buggy mess that stopped buying AC games and Unity has given no reason to change that view.