Mmmm... the siege units tend to be very well... paper thin in defense and don't they usually go RIGHT after the planet? as for the flak, utility and starbase constructor I'll see what I can do.
Personally I never use mines myself, I always found it to be rather tedious to maintain them and another thing is most of the time pirate ships invading or enemy fleets invading tend to shoot them down before they even have a chance to do the damage they are intended to do. Also another fact I don't like about mines is you get enough of them they impact the games ram usage which slows the game down considerably when it comes to mid to late games. I nix the mines, though... here is a thought for a ship ability, have one of the combat utility ships spawn a homing mine during a fight or a few mines, maybe 2 or 3, be a good way to counter an enemy capital fleet if you have nothing else available in the grav well at the time the enemy fleet invades.
going back to the siege unit, maybe giving them a bit more health hull and shield wise would help a bit, maybe a bit more armor too, though again the issue pops up with them going directly for the planet instead of the structures/modules around the planet.
Starbases, I can understand not wanting to do them right away, but just make sure they have the null points for weapons that way they are usable at least, Can't really test the effectiveness if they don't fire again after I load a saved game. It's confusing to find there abilities but once I know which ones are which I got no problem.
About the last model I send you, the incomplete one, I can send the complete one but that's if your gonna use it, of course I also gave you a candidate super weapon model which you are free to add on too and make it more mean looking or as the Nephilim canon shape, make it more angelic like, tries to sort of duplicate that kind of look with the rings though it didn't come out quite like I wanted, I just could figure out how to do it exactly.
and for now I'm out of ideas 