The main strength of the Fallout armor is that it's design philosophy is
cheap and
reliable.
You have a frame that is structurally designed to carry heavy loads and is still relatively nimble by
nature.
There's not a lot of complicated technology involved in Fallout Armor. These are things that can somehow still work
after a century of wear and tear in a radioactive wasteland.
They are cheap to maintain, cheap to produce en-mass, and versatile.
For an army they are
overpowered.
Even if their logistic profile is a bit larger than the average soldiers, the advantage they bring would make it worth it on just that alone.
And that's not even touching how versatile something that just
increases carrying weight,
endurance, and
strength can be from a footsoldier perspective.
You can make so many modifications to a loadout, and the role they take from this alone.
You can literally sneak around in Fallout while wearing PA.
While it cannot boost your speed by a crazy amount like other WH40K PA, it can give you more strength and endurance (with ease) while allowing you to have full functionality.
It doesn't take away from human mobility. You won't be any slower than it (unless you are superhumanly fast and outrun it in the first place).
The points you brought up are fairly nickpicky. There are a lot of Imperial units that prefer mobility.
The Guard is
not one of those. And not to the point they would reject something like the Fallout PA to obtain lighter armor.
Also, again.
This isn't something to replace SM, Sister of Battle, or MJOLNIR armor.
This is something to boost the ability of the common soldier. Let them bring out heavy weaponry.
Let the casual human duel wield bolters or wield a heavy bolter solo without vehicular support (This thing can carry Vehicular mounted chainguns for god sake, that's
an insane boost from a human perspective).
A faction with weapons that make armor irrelevant (Necrons for example), would
curb stomps the Guard regardless.