The problem is the balance, rush strategies are legit, but they should be able to be countered by other plays than rushing yourself.
Fundamentally they have the resource costs way out of wack, there are not enough defensive advantages.
Assault ships should cost alloy not minerals.
Starbases should cost minerals, not alloys.
This would mean that the attacker would have to spend alloy on warships and transport ships, the defender only warships.
It means starbases could be built as well as warships rather than instead of them during the early game.
The return on investment from colonizing is way out of whack. The dissembled ship should provide +2 unity, and colonists should produce 4 food each not 1. This would make them less of a liability since the food production would offset their consumer good consumption and the unity output would give a strategic advantage in the race for your 1st ascension perks. In addition, I'd double, the tech produced by establishing the 1st colony, so that even a rusher would benefit from making a colony before starting his rush, giving a tough choice to make.