As it has been since A3A was released, the Infantry Showcase being unplayable on Veteran+ is such a glaring, massive screwup that I cannot believe that it hasn't been the primary focus of A3 development. IMO, all development should be stopped until this gargantuan f**kup is completely fixed. If expert A2 hipsters can't have fun playing the Infantry Showcase of a supposedly infantry-focused game, then who can?
The friendly AI in the showcase are so woefully inadequate that they may as well not even be there. 90+% of the time, the player is killed by enemy he cannot see, often through vegetation. Even if he sees them, it is very, very difficult to neutralize them because of sun glare, inadequate weapon optics magnification, poor illumination on shadowed, south-facing slope, excessive recoil of weapon, etc. etc. If enemy AI spot you from any range and you don't have them directly in your sights and are ready to fire, you are dead, period. I often feel that BIS is playing a cruel joke on players with that mission on harder difficulties, even though I know that cannot be true.
A good QA/QC benchmark for playability is that if a moderately-skilled player cannot consistently finish a mission in 10-20 tries, the mission, or the AI skill/shooting accuracy, must be tweaked. This is a "realistic" game, not a hardcore VBS2 milsim, so it must be fun to play at all difficulty levels.