I like using The Elder Scrolls as a good example of how I view species balance on Aurora. Each of the races in TES has their own distinguishing traits, most of them based on their resistances to their natural climate and thus resistance to most natural hazards.
Humans are essentially Imperials and vice versa. They're milquetoast for the most part, and their strengths comes twofold: versatility and being socially well established. They have no notable strengths and no notable weaknesses either, they're a good starting point to make a character and they're not very complex to play -- which often means it's much easier to make an Imperial character that is well-defined and effective. You can still play as a human/Imperial and experience the game essentially the same way.
Tajaran/Khajiit have some similar details as well, they're both fast and agile. Fast-er and more agile than humans/imperials. Speed is certainly important with certain playstyle focuses, but because Tajara/Khajiit only have notable strengths in that category and weaknesses in almost every other, it limits their strategy and ability to survive certain situations.
Any strength can become a weakness in any case if an individual's strength is deliberately turned against them, for example. Breaking the legs of the Tajaran makes them go zoomy zoom zoom no more (sure, this also works for some others), and they're far more vulnerable to ballistic gunfire than anyone else, really. Especially if you play M'sai and get a cluster of buckshot to the legs.
The Unathi have burst-sprints and mild brute resistance as their strength, allowing them to mildly resist gunfire and melee weapons. However, they get wrecked by freeze rays (seriously, I've killed someone in an Unathi breacher this way), other types of gassing, acid, and lasers.
Skrell have limited psionics but are expected to have the greatest psionic development in those types of rounds. They've a better headstart than most, but it doesn't count for very much.
IPCs are IPCs. They cannot bleed out, they feel no pain, have some of the best up-front resistances, have absurd grappling values but get wrecked by EMPs pretty much instantly.
So what do humans have? None of those weird attributes. And they shouldn't. Presumably, humans are meant to outsmart and get the jump on their opponent. This isn't too different from when SS13 had no xeno races. Humans are generally flexible and have no notable weaknesses so that they can implement virtually any robust strategy and keep a tool of every kind in their pocket for any situation. Humans are much hardier than Tajarans but not as hardy as IPCs, so getting hit is undesirable but not round ending like for a Tajaran. Human sprint speed is not great but at least it's not like the Unathi who can only run 10 tiles in a couple seconds before wearing out.
Humans flat out play better as survivalists, using cleverness and their knowledge of the situation to their own advantage. They're physically and socially capable at every situation but must use appropriate tactics for each. If it sounds difficult: good. Nothing worthwhile to play should be easy and boring.