Broken Trust is a very asymmetrical battle. The Defender has the Letter of Marque (if you're playing the campaign) or a bonus Artefact (if you're not), but also a traitor in their ranks. The Ambusher has their warband split between two battlefield edges, which could be a strength, but is definitely also a potential weakness.
I had a lot of different ideas for approaching the "Traitor In Our Midst" rule.
I considered having the traitor be selected at the start of the battle, and then revealed at the opportune moment. That would have been cool, because revealing the traitor at the opportune moment would be fun, but it would also involve additional book-keeping, and could lead to a lot of unpleasant traitor experiences (like the traitor having to flip early and being really badly positioned, or the traitor dying to deadly terrain before being revealed).
I also considered having the traitor be random, but having the player choose when they would be revealed. Ultimately, I ended up deciding that would lend too much power to killing off weaker units so that you could increase the value of your traitor. It seemed a bit too gamey for what I wanted.
I ended up settling on a random traitor at a fixed time. The benefits of that are that there isn't much meta-gaming strategy to it, but it is still predictable.