I've done it myself. It's because you don't have to pay fuel for anything except teching. You don't really need the medic or mechanic side techs because you can repair and heal without them they're just an added bonus. It's also because they seperated the teching on the flak HT
But you have to pay the full tech to build a medium.
Which faction gets a medium out first isn't a balance issue, it's a gameplay one. It's not a consequence of disproportionate teching costs (they're all roughly the same), it's a consequence of how much fuel you spend on other things and how much fuel you're accumulating from your map control.
If you're regularly being rushed by mediums to which you lack a swift counter, you're either not holding your side of the map or you're spending fuel on light vehicles that you aren't turning into a map control advantage.