So... I'm a complete newb. I only have 3 years of battle dawn experience. After all that time, I still haven't figured out what is so bad about mixed chassis. As far as I know, if an enemy alliance knows that you use infantry, they will build anti-infantry and easily pick you off.

Its quite simple if you understand how battles and weapons work. Each weapon is made for a certain chassis (concussive for inf, beam for vech, and explosive for tanks). They do reduced damage to the other chassises.

Concussive Weapons:
100% damage to inf
50% damage to vechs
25% damage to tanks

Beam Weapons:
25% damage to inf
100% damage to vechs
50% damage to tanks

Explosive Weapons:
50% damage to inf
25% damage to vechs
100% damage to tanks

Each weapon also targets troops to attack by what it will do the most damage against. So if an alliance is split (like an admin build), all the concussive troops will attack inf first, all beam troops will attack vechs first and all explosive troops will attack tanks first. In this scenario, all the troops attacking are dealing 100% damage.

If you have one chassis, say infantry, the concussive troops will do 100% damage, beam troops will do 25% damage, and explosive troops will do 50% damage. By staying in one chassis you are able to take less damage. Also you pointed out that they can build troops purely against your chassis. However that will leave them vulnerable to a chassis change. For example, if they are infantry with beam weapons you can switch to an infantry chassis yourself and all their troops will do 25% damage to your troops.

It is as Abeyant says. Also you could go through the forums and search out the hundreds of threads where we discussed this in great length.

I recommend you to fill an application in the AANC thread then.