First off, you can't go whining about one side being better than the other during SD because most better players prefer the challenge as aliens, while most humans are hopeless camping newbies with no clue on proper base building. In a good base, aliens NEVER have easy access to an armoury, and even if they do, there would always be at least one builder ready to repair, even more so during SD.
The overmind is, in fact, as important as the reactor and the armoury together for an alien team, and it is also a much easier target than a reactor, a single kamikaze saw can kill an overmind in less than 5 seconds, while the reactor always takes at least 2 or 3 kami runs from a marauder to kill, if theres no builders around repairing. A couple of humans with rifles can hold up a base with good defense, however, once the overmind is gone aliens have only the remaining big aliens left as offense AND defense for their eggs. If you did lose your armoury, 9/10 times you will be able to kill the bigger aliens when they try to attack with help from your defensive structures (that would be chasing them as they retreat, not hugging turrets hoping they do all the killing).
Theres also several servers that allow Overmind and Reactor rebuilding during SD, this does favor aliens a lot, but most servers don't have this.
Ksempac is right though, since humans mostly camp and let aliens pin them down, once SD hits the human bases often don't last long.
