It appears you are only allowed to have a valid opinion if:
2) You play both factions
That's exactly right. In my experience, no person who plays one faction only has a clue when it comes to balance.
It's not because they're stupid or bad people, it's human psychology. If I play one faction for too long, there are elements about the other faction that seem OP or hard to deal with. This is normal and the only solution is to play the other faction and see how people counter you doing the same thing. Only in this way can you get a more or less balanced view of a unit's strengths and weaknesses.