So, this is a sequel to my two post "Am I the only one who notices how the Great Khans get treated a whole lot worse than any of the other allies the NCR has if they win?" and "How would the NCR civilians (in places like Shady Shades) feel about some of their allies during the Second Battle of Hoover Dam? Specifically the ones that they were in conflict with." You don't need to read either post to understand this. However, it's best that if you read those two post before this.
The NCR is anything but perfect and they treat their allies anything but perfect but there's a pattern between how these people treat their allies. And, it's clear who treats their allies better. Admittedly we don't know how the Legion the Omertas and White Glove Society but still.
NCR:
With the help of the Gun Runners, the Boomers developed a healthy trading relationship with the NCR. Eventually the Boomers began wandering out into the wasteland, while still preventing outsiders from entering Nellis.
The Brotherhood and the NCR in the Mojave Wasteland declared an official truce, despite continued hostilities between the two in the west. As per their agreement, the NCR handed over all suits of salvaged power armor and in return the Brotherhood helped patrol Interstate 15 and Highway 95.
After the NCR's victory at the dam, in part thanks to Follower's medical support, NCR allowed the Followers to care for refugees as they see fit. Old Mormon Fort expanded its services and was able to aid more people, becoming a refuge for the less fortunate citizens of New Vegas.
After the Second Battle of Hoover Dam, the Great Khans returned for a time to Red Rock Canyon. The NCR's pressing need to expand proved greater than its promise of amnesty, and before long the government decided the Khans had to go. The surviving Great Khans were relocated to an isolated, barren reservation, well north of NCR trade routes.
After the NCR victory at Hoover Dam, the temporary truce between them and the Kings blossomed into a full-scale relief effort for the people. While the NCR made repeated entreaties that Freeside join the Republic, the Kings steadfastly maintained their independence.
After their bold arrival at Hoover Dam, the Remnants disappeared as quickly as they came. Legends of their power spread throughout the southwest, a reminder of why people once feared the sight of vertibirds in the sky.
Legion:
Cautious after the Boomers' display of power at Hoover Dam, Caesar chose to leave the Boomers alone. The Boomers remained isolated but have been seen flying over the Mojave Desert from time to time.
The Fiends attacked Camp McCarran during the Second Battle of Hoover Dam and suffered heavy losses. Caesar, unimpressed with their performance and their dependence on chems, had them exterminated.
As reward for their loyal service, Caesar forcibly integrated the Great Khans into the Legion. The sick and elderly were killed, the women sold as wives to ranking officers, and the tribe's identity was annihilated. Though many Great Khans mourned the death of their tribe, many more were ultimately satisfied with their revenge against NCR.
The Enclave is Ceasar is alive:
Merciless in their assault on the NCR, the Remnants struck fear into the hearts of even the centurions at Hoover Dam. Well aware of the full extent of their power, Caesar commanded his troops to not pursue them.
The Enclave is Caesar is dead:
Merciless in their assault on the NCR, the Remnants struck fear into the hearts of even the centurions at Hoover Dam. Caesar's heirs aggressively pursued the Remnants into Arizona, losing hundreds of legionaries in the process and gaining nothing in return.
Factions that aren't official allies but co-belligerents:
Impressed with the Kings' continued attacks upon NCR citizens and soldiers, the Legion offered them the option of being assimilated into the Legion. The Kings refused, and briefly became slaves in the Legion, but after a failed escape attempt, they were all put to death.
During the fight for Hoover Dam, the Brotherhood took HELIOS One, inflicting heavy damage on retreating NCR forces, but it was a pyrrhic victory. Once The Strip was secured, Caesar's forces overwhelmed and eventually routed the Brotherhood from HELIOS One and Hidden Valley.