What is rare is finding a Union hero in a Western.
Oh God. Here are a few off the top of my head:
Wyatt Earp in all kinds of Westerns. He tried to join a Union regiment but was too young. Virgil did serve in an Illinois regiment.
Wild Bill Hickock in many Westerns.
Linus and Zeb Rawlings, How the West Was Won
Nathan Brittles, She Wore a Yellow Ribbon
Kirby Yorke, Fort Apache
and Rio Grande
Seth Adams, Wagon Train
Paladin, Have Gun Will Travel
Lucas McCain, The Rifleman
Cord McNally, Rio Lobo
John Dunbar, Dances With Wolves
Colonel Thomas, The Undefeated
That's because the classic Western is about a racial mythology.
So you claimed way back up thread and were given plenty of contrary examples.
Wyatt Earp's Republicanism was a deep dark secret for the overwhelming majority of his screen incarnations, till Tombstone, definitely not a classic Western. On the other hand, Doc Holliday, Southern gentleman, has taken equal billing in every one. Wild Bill Hickock's Civil War experience was never part of his screen persona, unlike Jesse James.
Some of the rest of the list I'm not familiar with, but thinking Dances with Wolves was a classic Western is imperceptive, to put it kindly. Ditto thinking that the Unionist in Major Dundee was a hero (the Confederate played by Richard Harris is the martyred hero.) The view that the power-mad Henry Fonda in She Wore a Yellow Ribbon and Dundee are natural fits for Union officers appears to come from sort of notion that the Union side was a bunch of crazed zealots on an unholy crusade against sane people. Given that John Wayne in The Undefeated unites with the unreconstructed rebel Rock Hudson, The Undefeated is definitely not a Western that challenges the racial underpinnings of the classic Western.
Just as before, the supposed examples in refutation don't refute. Quibble about "rare" but Unionists are rarely heroes by comparison with Confederates, and when they are they are usually happy compadres with Confederates with whom they have no differences in principle. Or still on active service in the national army. How could you expect a serving officer in the postwar army to be a Confederate veteran?