Balance can't be perceived and examined, or applied as a single solid object. It is a complex mechanism, and sometimes one part doesn't make sense when you compare it to another part, but it still works when put in a game.
Stronger than average ships, especially bigger ships, give a nice challenge to player. Which is very good. Though regarding Valks - they aren't that hard to defeat, and the main reason is that they are pretty straightforward faction. Some ohter factions with a bit weaker stats on bigger ships, but more different approaches to combat, give a lot more trouble to me.
Another thing is that in end game, when you have all those bigger ships, balance already doesn't work. Will you roll over the paper tank with Merkava, Abrams or T-54? Doesn't really matter, though obviously every vehicle has its own strenghts and one obviously will be stronger in certain situation.
So from a point of fighting against Valks I prefer them to be stronger.
When I play with Valks they are a bit different from other factions, but overall it is a minor thing and mostly how pleasureable my experience depends on how I play, and less on how exactly balanced ships are in terms of numbers.
And the last point, which gives most issues with balance, is when you mix weapons and ships from different factions. But given number of factions I don't think it is realistically possible to balance such thing, only limit to some extent those mixes, which will break the game.
I personally think that the best measure of balance is how content mod creator with his own creation. Most of the time end product will be good. Players can give their opinions from their point of view, but mod still will be a creation through the eyes of the creator, so the most important opinion is one of the creator.
And the funny thing is that since SS 0.6 balance revolves more about logistics. So using biggest possible ships most of the time isn't best decision, which overall makes them balance wise weaker.