Ok, I'll revise this a little. Let's say each race is equal to themselves, better in offense but worse in defense to one of the races, and better in defense but worse in offense to the other race.

In a two player, game, you will either have the same race fighting, which is equal, or two different races fighting where there are three possibilities. Either both races are strong in offense and weak in defense against each other, which means that ships on both sides will be destroyed faster, but it will still be equal. The opposite can be the case, where both sides are strong in defense against each other. This will cause longer battles, but still be equal. The third is that one is strong in offense while the other is strong in defense. In this case, they even out.

It can still be interesting for multiple factions. When two empires are fighting each other, the above applies. Let's say I'm fighting a race and we are both strong in offense against each other, but that faction is much larger than me and has better technology so I'm at a disadvantage. What I need to do is get an ally who is strong in defense against this race. Now when both of us are fighting together in a battle, my ally who is strong in defense can draw the enemy's fire while my ships that are offensively strong can deliver the damage.