And AT guns disproportionately affect offensive operations in phase A because they force the other player to commit resources to defensive assets, hide their tanks from AT attack and generally protect their armoured assets from AT attack.
Bollocks. AT guns are slow-moving, defensive weapons with a wide range of counters - from infantry close-assault, artillery, airstrikes or fire support vehicle suppression, with a very limited window of reliability (inside gun range but outside HMG range), which anyone who uses them can attest to. False comparison is false.
Yeah, if an enemy deploys a unit, you have to counter that unit. That's how this works. The HS129 is not disproportionately difficult to counter. Again, if they don't deploy one then you don't need to deploy a counter. An unvetted HS 129 is not disproportionately more effective than an AT gun. The AT gun can often be more effective, depending on the situation.
125 points; 25% of the starting standard points loadout, or 2 ticks (1 if you're playing the mediocre Polish division)
Can a HS129 be outflanked, be pinned in position, be destroyed with indirect fire, be assaulted with infantry? No.