"Balance is messy and as far as I can tell it is currently done by a couple of the top players having a discussion about their intuition of what is op and what is not."
Yeah, I think that's how we wound up in the current situation regarding T3 land. Assault Experimentals used to be in a place where they were niche units that complemented T3 land, but their utility dropped off in higher skilled games.
So they changed that, and now assault experimental units stomp on the role of T3 land formations, effectively replacing a large part of the game on some maps for some factions.
People that understood the complexities of balancing a game set up a system, and when the hand off to the FAF team happened, the intentions and understanding of how things were supposed to work were not conveyed. The FAF team then made a change that on the surface seemed very reasonable, but which had unintended consequences.
This is further exacerbated by the differences between the kinds of maps you find in ladder versus the type preferred in team games, where map control is often less of an issue. Maps like Isis which were once dominated by T3 formations are now dominated by T4 assault experimentals (unless the game goes late and the map closes completely), and replacing large diverse formations of many different units with a single unit is clearly not good for gameplay.
The point is that there is more to balance than just knowing what is OP and what isn't. There is vision, and If you lose vision, its easy to mistake the role of a unit and make choices that have far reaching unforeseen consequences.
How do you keep Assault Experimentals from stepping on the role of T3 land? You make them weak for their cost, but fast to build, so they can be used to take advantage of a large reclaim field quickly, or be rushed in a surprise attack. That's how you do it. It doesn't mean that they're underpowered.
If you don't understand that this was the vision behind them, and then make changes, you wind up undercutting a lot of other units and reducing unit diversity in the late game.