The Jiralhanae are, buy-and-large, a species full of violent idiots.
This is also true of Humanity.
When the Covenant had found the Jiralhanae, they were amidst their second industrial age. The reason why it was their second one? The Jiralhanae began to become space faring only to nuke themselves in one of their perpetual states of war. The nukes brought them down to the stone age once again, or at least the bronze age, and then they had to invent their way back up to being technologically advanced.
For every 100 Brutes you have, there is probably 1 who is smart enough to realize that rage itself isn’t going to solve the issue at hand. For every 1000 Brutes, you have 1 who is smart enough to perform engineering tasks.
The problem is that the rage the species has is intense and they often go to blows rather than talk things out. and think things through.
The few brutes that are smart enough to think effectively and lead them? They are extremely rare. Hence why you have guys like Atriox and Pavium who are strategic masterminds and engineering geniuses.
Atriox has made a reputation for himself of wanting things done a certain way and his orders to be followed. Those who break those orders tend to have their heads smashed in if they screwed up big time, likely what happened to Voridus after the fiasco with Awakening The Nightmare.
The three Spartans in question that you are referring to are likely Jerome, Alice, and Douglas. They do not have much experience with skirmishing with Brutes as most other Spartans. Not to mention their Mark IV armor is pretty deficient compared to the other models that came after, what with lacking shields and all.
As for what happened to the UNSC Infinity? Well it appears that their shields were down in that cutscene. Given how A.I.s betrayed Humanity, it could be that Roland was a sleeper-agent of Cortana and when the UNSC Infinity arrived; he sabotaged systems which resulted in the embarrassing defeat of the UNSC Infinity upon arrival to Installation 07. But then again, we won’t know until a book or story DLC comes out to explain that bit of information.
Atriox’s Banished are just that. The Banished. They aren’t accepted into regular society on most worlds. Though Atriox and others call Doisac their home, they are fugitives. Even Let 'Volir, the Shipmaster of the Enduring Conviction, was declared dishonourable for offering his mercenary services to The Banished.
And why the UNSC wouldn’t respond to raids for years? They likely thought it was Covenant raids and given the fact that they were at war with the Covenant on a highly-defensive front, they likely wouldn’t be able to do much to target them.
In fact, this was CONFIRMED in the Phoenix Logs of Halo Wars 2. There were a few logs from an ONI investigator that followed these raids but came to either dead ends or was told by his superiors to focus his efforts on what they saw as more important tasks than the occasional raid on an outer-colony world.
That is kind of how fictional writing goes.
You find things to base your creations upon and you expand on it in your own way.
Take a look at The Argonians from The Elder Scrolls, who’s culture and society are based off of the Central American cultures of Maya and Azteca. Or the Imperials of the same series, who are based off of Romans.
The Jiralhanae are simply a case of what would happen if apes evolved on another world. However, unlike Humanity which held more moderate values when it came to violence and rage, the Brutes evolved to retain their primal rage and even reveled in it.
That is kinda the entire point of a power-fantasy FPS game.
Master Chief isn’t your ordinary man.
He is not only a Spartan super-soldier, but THE BEST of them all.
The reason they hype up the enemy so much is to make you feel even more badass for killing them all while everyone else has a struggle.
You are meant to be the icon of the front-lines and inspire others on the battlefield.
It is power fantasy 101.
What made Doomguy different from everyone else in the older Doom games? Nothing except his unwillingness to die by the hands of Hell.
And then when he made it to that alternate dimension and got into the Divinity Machine, he became literally God incarnate and unkillable.
That is part of the story, the mystery of it all.
If you knew everything from the get-go, then the story would feel flat and have almost no motivation to progress forwards. Unsolved problems and mysteries is a way to pique interest and get you waiting for the next chapter to answer said questions.
Atriox went to the Ark for technology and a map of the installations.
He is now specifically at Installation 07 in search of The Endless.
Given from what we know of Doisac’s destruction at the hands of Cortana and the fact that The Endless are given some implications of having control over time itself to some degree, Atriox’s motivation might very well be to undo the destruction of his homeworld. His efforts might result in him getting what he wants or for him to be struck down for what The Endless might view as insolence.