I'd say Balance has very little/nothing to do with the complexity of a core rule set. Balance, in the competitive sense, is about the relative power levels of factions within a system. For example despite very simple rules chess wouldn't be very balanced if white pawns could move an extra square.
Of course as a bit of a counterpoint may be that is the "complexity" of AoS right there, the rules are simple but the real depth is creating equally enjoyable scenarios for both players (because, being a fun experience for both players is the real balance). The game is coming from a company that prides itself on "forging the narative" would that really be a surprise.
It's all well and good drawing comparisons to computer games but remember WoW and others are fixed systems you can only play the way Blizzard or whomever lets you play. Wargames are not fixed the rules are only guidelines you can actually alter them however you like. It's very telling that all Tournaments come with an extra set of rules for play, I believe the PP one weighs in at 14 pages at the moment.
In fact I think this ties in with the Infinty example which has complex rules but rather simple army list mechanics. So tricky to learn but easy to balance which is good for more serious/competitive gamers. AoS is very easy to learn but more complex to balance so not so good for the competitive crowd but rather good for gamers who just want to play, younger player for examples who in the early years of their time (in Wargaming) won't even have a concept of "balance" in the competitive sense.