Even symmetrical maps can be unfair, for example Aeon being weak in navy unless there are narrow chokepoints where destroyer shots can't be dodged. Or how some maps have mexes near the water that are vulnerable to zthuees. So it would be a mistake to assume that symmetry automatically means maps are fair.
In terms of differences: I would see the differences as being small rather than large. I wouldn't give one player 2k extra mass reclaim and the other player gets easy access to 5 extra mexes. That might be "fair" but that's not the kind of assymetry that would be particularly interesting. I'm not suggesting one player has 1 chokepoint and the other player has to defend 4 chokepoints on their side of the map. Or that one player has very fast access to the water to establish a naval lock but the other player can capture a civilian T2 power plant next to their base. Just that the layout of hills and chokepoints and ridges would be very different on either side of the map, that if you look at both of them you wouldn't think they have anything in common, but would be intended to give each player similar opportunities to defend and attack on either side of the map. The mapmaker should try to make things even in terms of: how much time it takes to get to key locations (mexes, reclaim, chokepoints, good places to establish a large naval yard, etc.).
IMO "fair enough" for use in ladder matches would be something like 45/55. Meaning that if players are evenly-matched, instead of winning 50/50, they would win at least 45% of the time. Getting a "bad" spawn on such a map should be no worse than getting a "bad" faction matchup. And ideally even with the differences it would be more like 50/50, because over time if one start location is shown to be "bad," the map could be slightly tweaked to benefit that spawn, pushing it back towards being 50/50.
If a mapmaker is making an effort to create a balanced assymetrical map, and gets it to a point (by testing it before it is included in the map vault) where it is roughly 45/55 in terms of fairness, that should be enough to let it into the vault to allow for broader use/testing. Over time, as the map gets played (which assumes people want to play it--if people don't play it, there's no problem) and a meta develops, the mapmaker could tweak it to increase fairness. The goal would be to make a map that is 50/50 but it shouldn't be locked out of the vault just because it isn't quite there.