Thier armor isn't that bad either the IG that is.
Their armor is pretty horrid actually. You need to remember that the IG is explicitly designed as a planetary expeditionary force. The local PDF is supposed to be the static garrison force, sufficient to handle the expected threat profile of a world. The IG is supposed to be the offensive and intervention force; deployed to take enemy held territory or defend territory under extreme threat.
What this means in practice is that the IG has to ship its armor between planets along with spare parts, ammo, fuel, and all of its other logistical needs. What that means is that IG armor should be designed to have the smallest logistics cost possible.
IG tanks should, given the IoM tech base, be all electric and using fusion for power. Use solar power to distill and then split water into hydrogen and oxygen, fuse the hydrogen for power, arm the standard tank with Las weapons and an EM rail gun. Take iron, either fusing up to it or extracted from the environment/sourced locally, shape it into rail gun rounds. Las weapons for anti-personnel work and for active defenses (shooting incoming rockets, mortars, and potentially even bolt rounds or lower flying aircraft out of the sky), the rail gun for heavier work. Ideally use a Las shot fired immediately before the rail gun to burn the atmosphere out of the way before the rail gun fires so that you can avoid air resistance. The sole logistics cost of such a tank becomes delivering the tank to the battlefield, everything else can easily be sourced locally and it is good enough for most IG needs.
IG artillery should be essentially the same, save using a more powerful railgun to deliver bigger and better shells. Anywhere with a breathable atmosphere should have all of the needed molecules in the air to turn into at least passable HE. Sure, you probably need an off world supply for more exotic ammunition but allowing organic local sourcing of your primary type of ammo is a huge reduction in logistical burden.
Then you have the fact that the first thing any IG force arriving at a planet should do is have the Naval vessels kick out an entire orbital surveillance, communications, and strike grid. Yeah, what can be economically deployed won't do squat against shields and peer competitors can probably down your satellites pretty easily but in both cases it requires them to have and use the needed resources and if they don't, you get full coverage and strike capability. Drop a few thousand bolt rounds, for example, onto the enemy artillery park timed for your advance. Or use an X-ray laser to do some precision assassination of anything tactically relevant. Yeah, you might not be able to strike down the Warboss with an orbital las strike but a Nob is a different story.
Granted, you also have the idiocy that is IG organization and structure. Take a few world's in a subsector and turn them into IG training worlds. Anyone who wants to join the IG is shipped to one of said worlds where they are then organized into regiments and trained to the same standards, preferably with a rotating cadre of Space Marines overseeing the training conducted by veteran IG non-coms. Train the IG, at least on the sub-sector level, to the same (high) standards with the same equipment while speaking the same language and with officers trained to the same (consistent) doctrine and standards. Incidentally, this also breaks the potential for regimental loyalty to its planet before the Imperium. Logistically, it makes it far easier for forces to be consistently and timely deployed as needed.
Then you have the lack of armory/forge/logistics ships at scale. One of the most common vessels in the IoM should be basically a mini-forge designed to produce everything that an IG army needs using locally sourced materials. Basically, one of these in system should be able to keep an IG army of, say, ten million fully equipped to standard for say a century using nothing but locally sourced materials that it can gather itself (i.e. asteroid mining, for example). The Imperium has the tech, they even build ships able to do just that occasionally, they just don't bother to do it at any kind of scale.
And for planetary strike, take one of those armory ships, give it some rail guns, and have it mass produce proximity fuzed bolter rounds. Once an enemy battlefield or fortification is identified, just launch a few dozen million of those on a course calculated to deliver them to the needed grid square. Sure, it will do jack to shielded targets but most things aren't shielded and you can always just keep the iron rain crashing down around the shielded area for a few days/weeks/months/decades at a time. Counter battery fire on a void shield equipped vessel a few hundred thousand kilometers distant and constantly moving in random directions at variable accelerations is basically a pipe dream.
But then the IoM doesn't actually appear to have any dedicated planetary siege vessels. Its naval vessels are all primarily designed to engage one another in the void with any planetary strike missions either being very strategic (at a minimum removing an entire city) or requiring the vessel to be absurdly stupid (ah yes, we must bring the cruiser into atmosphere and park it in orbit over the battlefield for a full day before it can strike any of those puny ground-pounders with anything approaching precision). Take a cruiser sized hull and equip it to fire what are basically bolt rounds in the millions along with the needed computer support to handle relatively accurate targeting from trans-lunar orbit. Give it some kind of surveillance parasite craft that can be sent in ahead for mapping/targeting purposes, and then just unload on enemy grid squares. Drop a million bolter rounds into a square kilometer with them all fused appropriately and your problem is solved. Even a basic bolt round should be able to handle orbital re-entry, already has a rocket engine for terminal guidance, and can already be equipped with all of the needed fuses and payloads. Again, "That's a nice artillery park you got there, shame it just ceased to exist."