It's not balanced for actual players. It might be balanced based on the number of characters created however it is difficult to confirm. Log in to WOW Classic TBC Gold check M+ keys or pug raids. I'm playing max level characters from both factions. The skill level is quite different, but it's difficult to quantify from a distance and could be a consequence of all those who are interested in playing seriously rerolling to the horde.
There are people who defend Blizzard for this reason. They make weak arguments, such as "if players want Horde and that's the way it's going to be, it's impossible to hinder players from playing Horde" however it's quite possible.
Blizzard might allow you to switch factions from Horde or Alliance without cost, but they are just too stupid to do so. "This wasn't a possibility in the initial TBC" According to their reply, while they are selling 6-month game time that gives you a Mount and/or Pet that wasn't included in the original TBC which completely breaks immersion because it's the only mount that has modern animation.
One of the main reasons are that they're unable to buy WOW TBC Gold return the rabbit to the hat. It's a permanent change to the game. They also fear that cross play could cause the name of the horde/alliance being lost, which is an issue considering how much marketing it brings in.