Now let's talk about the Guardians, they allow the alliance to deal with threats the Mobile Infantry's fire power can not.
Guardians can operate solo, in strike teams of three or up to six at once. Rarely do they operate in much larger numbers than that unless it is a massive operation.
Guardians main strength to this Alliance is they are a Super Soldier equivalent that can self ressurect as long as their Ghost isn't killed, this makes a cautious and competent Guardian nigh unkillable since they can rez even from complete atomic destruction.
This puts them at a significant advantage, they have several attempts to kill someone they only need to beat once.
Their mental fortitude would also make them invulnerable to the psychic shenanigans of a Psyker due to the biggest threat aka Mind Control not being effective, they also are powerful enough to resist the Telekinesis of Psion Flayers three of which are enough to move Phobos, so TK is out of the equation.
Their ability to defy fate and preordained destinations literally shutdowns Precognition.
Even BFR for example to the warp isn't possible because Oryx who literally BFR's his opponents is soft locked from doing that to Guardians, giving them resistance or even immunity to forced BFR.
Guardians will be perfect for being used against high priority targets such as Psykers or enemy Commanders, with sufficient ground support they can also hold lines indefinitely.
If for example one line of a thousand Guardians get killed by being overrun, if the line is taken back then all those Guardians are resurrected again.
It would take very accurate heavy weapons fire to their Ghosts or very large radius explosives to Perma kill a Guardian.