TLDR: Ancient Humans a little OP and the Forerunners under powered in practice, though I cant quite identify why this is.

The Ancient Human + Ancient San'shyuum alliance seems to always be stronger, by a considerable margin, than the Forerunners in my games.
I am playing with a quite large pack, but other than getting a third civic at game start, the factions are unaltered from your presets.
In multiple games the Forerunners have lost Maethrillian to other empires (I dont think it was ever the Ancient Humans, but others) by year 75. And for some reason in about 1/2 of my games the name is changed to just Maeth for both the system and the capital.
So, this might not be a case of the Humans being totally OP (though they are for sure more powerful than the non-Halo factions in the galaxy and are often the top scoring once the GalCom forms) as much as it is the Forerunners having some sort of issue, at least in AI hands.

As a non-AI example:
Playing my current run as a custom Forerunner (regular Forerunners are also in the galaxy) on Installation 00, I'm currently fighting the Ancient Humans + San'shyuum and they are greatly outclassing me (Grand Admiral, I think I set it to no scaling, but it may be about 1/2 through midgame scaling), with both their federation fleets and individual fleets outclassing me slightly. Mostly, they outnumber me by something like 3 or 4 to 1.
This is very surprising to me as I have a large empire (only year 25, so everything is fairly early in development) while the Humans have less than 1/2 and the San'shyuum only have 1 world.
The San'shyuum themselves have less than 1/2 my total fleet power, the humans have about 3-4 times my power, but in addition they have their federation fleet, which is currently commanded by the San'shyuum and brings their fleet power to above mine and close to that of the humans.

I did notice that the Ancient San'shyuum trait gives much greater bonuses for admirals than the Forerunner trait, though that is mostly offset by Warrior-Servant leaders.


My particular case (me as Forerunner) might not be representative, but I do consistently see the AI Forerunners outclassed by the AI Ancient Humans, every single game I've played recently.
Which is a bit unbalanced, especially compared to the lore where the Humans were rivals of the Forerunners for sure, but they did not outclass them, especially in terms of territory.