Let's first consider the scale of this conflict. An army of 100 Ultramarines [Tactical Marines + Devastators with heavy weapons and Lieutenants] (and excluding transports), comes out to around 1877 points based off
this list (1997-120 cutting-out the Razorback), more than enough to replace some of those marines with vehicles, Primaris, Librarians, Dreadnoughts, Terminators. While that's crunch and not fluff, it can be used to get a sense of scale for this conflict: big. That's very close to the 2000 point limit for most tournaments, which is on the scale of battles including tanks, giant walkers bristling with guns (for Knights), air support, et cetera.
This kind of force represents a Battalion in Universe terms, and Standard Equipment for a 40k Battalion is heavy, really heavy.
The UM are one of the primary adopters of Primaris Marines, so in a "standard" force (whatever that is), I'd expect to see quite a few of them. Primaris marines are considerably more powerful than standard marines, featuring more sophisticated weapons, even faster reactions, higher strength and speed, the ability to literally bite through armor...
Power Weapons: Power weapons are encased in a "sheath" of energy which melts/destroys/ignores most matter and most shielding. Definitely enough to penetrate USIF armor and shields, since it's literal magic.
Psykers: Back at it again with the main reason 40k walks over most other scifi universes: magic.
Of course, part of this is the non-specific nature of this set-up. 100 Armored Infantry is so much more specific than 100 Ultramarines, because Ultramarines include:
-Tactical Marines
-Devastators
-Captains/Lieutenants
-Assault Marines
-Terminators
-Primaris
-Librarians
-Dreadnoughts
-Crews for vehicles, like Predators and Thunderhawks
-Crews for space vehicles, like Fury Interceptors and Battlebarges
-Their Primarch, Roboute Guilliman
All of those are considered "Ultramarines." An Ultramarine flies a ship, an Ultramarine fights on land, an Ultramarine mans Artillery.
Of course, some of those items can be immediately discounted: One Battlebarge goes
way over the limit of 100 marines, and vehicles can
probably be discounted based on the nature of this combat themselves
But even saying "Only Soldiers" still allows for most of that list, including the ROFL-stomp of allowing their Primarch.
As this conflict is currently written, the favor goes straight to the Ultramarines because they are able to take so much more of their stuff, including a literal Demi-god and literal magic-users, versus what is essentially a force of Tactical+Assault Hybrids with a few heavy weapons.