>>460
The biggest problem the IG have is their organizational structure, it is literally Napoleonic era. There is no official formation larger than the regiment (there is a lore reason for this, but it's fucking stupid.) This means that you jump straight from regiment level to army group (or crusade) level. This is horrifically inefficient, stupid, and has likely cost the Imperium entire campaigns. To put this in perspective, modern military goes Squad>Platoon>Company>Battalion>Regiment>Brigade>Division>Corps>Army>Army Group>Operational Theater.
IG goes Squad>Platoon>Company>Regiment>Imperial Command Elements (Lord general's staff, whoever is in charge of the theater.) This, far more than their tactics, which can be perfectly fine (see Elysians) is why the IG is such an inefficient weapon. It's mentioned that in large campaigns, regiments with complementary abilities will be mixed together, but the lack of anything resembling a proper chain of command means this is a shaky solution at best, if two regimental commanders don't get along they can screw up their entire front, and they have no direct commanding officer to oversee them and make sure that doesn't happen.
Imagine this as the strategic equivalent of having a mob of uncoordinated idiots running at a riot police barricade, while modern armies are an actually disciplined group with a clear command structure all the way to the top. Every Imperial regiment is expected to operate independently despite being purposefully overspecialized, combined arms is prohibited by doctrine. They ignore this when they have to, but by the Emperor it's dumb.