Babyface versus babyface main events do sometimes work, but when it is a triple threat and all three are (supposed) faces, that's a little tricky to pull off. The dynamic is just hazy, without a clear heel to root against. Of course, Roman Reigns is loathed by many, so he'll be a default heel on the night, but that's the exact opposite of what WWE are trying to do. He's meant to be the biggest babyface in the company. By booking him in a match with no heels, they are only going to make him look even less popular. He's got no heel foil to work over, he's only going to end up looking bad by beating on Lesnar and Ambrose. This match really needed a Sheamus or Rusev in it, just to enhance the faces and make some narrative sense.