I ask this question in good faith and without any intention to cause divisions. Regular Seton's players often have ideas about stuff being unbalanced which is complete news to me, and I wonder if these imbalances are genuinely game-wide or if it's just a peculiarity of one specific map.
I can see the reason for Seton's popularity, it is designed in such a way that every unit in the game is viable, and the appeal of this is obvious.
It seems to me that this unit viability has been mistakenly extrapolated to tactical and strategic viability. Although Seton's is well-designed in terms of allowing any conceivable tactic to be used, it is not representative of the wider game in many ways. For example it has a central chokepoint: in other maps this sort of feature is considered bad design and not what FA should be about. I appreciate that Seton's is different and it has a whole metagame associated with it, but that in itself does not mean that the rest of FA should be changed to accommodate it.
Seton's has 'gentleman's agreements', for example no mixing naval stealth and shields, but the rest of FA has no need for these. There are two opposing arguments that I can see as the reason for this. On the Seton's side, they would argue that Seton's allows a huge number of iterations of high-tech unit interactions and therefore they are best placed to see how the balance should be. The opposing view would be that Seton's is not representative of the game as a whole and has numerous limiting factors that influence which units can be used and how.
I'm no FA expert, and I'm certainly no Seton's expert, but I think that this is an interesting area to discuss and I would like to hear others' viewpoints.