There were problems noted with the heavy assault units configured for gas attacks due to lack of visibility and the general cumbersomeness of them in straight up skirmishes.
The configuration is still in inventory and they can be applied by willing commanders.
Notably the non gas versions are effective in skirms especially when applied with supporting Standard Units.
Once lifted from the Standard config the unit will generally stay in that role. Meaning that they are only sent into direct combat missions, as opposed to the standards where they are applied in patrols, sentry duties aswell as force application in combat missions.
Deciding to lift a unit into the role of a HAU varies between commanders as there is no strict enforced guidelines in place as of yet, and possible there never will be.
Essentially the HAU's are a reserve of heavily armed fast response hard hitters on standby.
Where as moving standard combat Units from other duties to respond to missions can leave areas under saturated in combat units.
Asthetic perceptions for commanders changes in general also.
The cloth uniform & soft cloth combat veil gives the perception of "lightness" or "softness" to the standard combat unit, merely throw away tool adorned with lethal apparatus.
The harder, shiny latex bodysuit of the HAU covers less, but gives a more domineering "heavy" or "Hardness" to the Unit. The latex gas apparatus, heavy weapon and tubing gives a machine like quality to the aesthetic. A menacing, death dealer.
In perceptions of the standards, studies have shown that the uniform is powerful to them, especially in large numbers. The dehumanising aspects of the uniform and accompanying training give the sense not of "sacrificing for" but more of a "sacrificing with for the mistress" hive mind mentality.
The perception of the HAU's in recent studies show that after training they take on more of a rogue or lone singularity mentality of "destroying for the mistress" rather than sacrificing.
The uniform with gas apparatus is the ultimate empowering tool to the HAU, seeing themselves as machines, or inanimate objects, their faces totally covered and they know they appear menacing to the enemy.
The heavy gas mask is also a secondary barrier between them and the outside world. Much similar to human behaviour in vehicles, the feeling of safety pushes the boundaries of danger closer, which is effective in combat.