There has been at least one CG where the side with the obviously better reward lost - and it wasn't even close.Fdev came up with the narrative and then added the shiney's and $$$ to make sure the pilot mercs always choose what Fdev wants in the CG's
One side was offering standard cash tiers only, the other offering the same cash tiers + a unique module.
Final result: slightly better than 2-1 victory for the cash-only side.
Another couple of recent CGs Frontier was accused of bias because one side had a route to obtaining combat bonds which the other side didn't. In both cases the side with the extra route still lost, though - if more narrowly than they might otherwise have.
There have also been competitive CGs with exactly identical rewards offered by both sides that have gone to a 4-1 margin or more.
Frontier are generally a bit more subtle than that: they control the narrative, so they can set it up so that either CG outcome progresses the plot in a direction they're happy to follow, and only put in competitive CGs at all when that's an acceptable outcome.
[1] I think it's worth noting here that player records at predicting which side will win a CG based on the starting setup are not more than about 75% reliable either. It's always easy with hindsight to find a reason why a particular CG was biased in favour of a particular side (my favourite excuse "the BGS tick was at around 12:00"); it doesn't count unless you can call it very reliably in the first few hours.