I consider AAgun and ATgun late-game specialist units. Extremely efficient for exactly one job, bad for everything else. You need techs to make them viable.
Consider a late-game situation. You produce four AAguns for 16 PUs. Before combat, they kill a fighter for 11. That means, you essentially paid 5 PUs to add four hitpoints and 12 firepower (or more on good terrain) to your army. How crazy is that? If you can afford to lose only inf in the first round, they kill another fighter. Insane value. ATguns are not as extreme, but still very high value, better than inf.
Here's the downside. They don't attack. Congratulations, you just built a fence around Moscow but lost all the important Barracks around Moscow. An important part of Defence is, make sure the enemy doesn't survive next to your army. And other techs (Production, Fighter-Range) are more urgent. That's why I consider too many AAguns or ATguns a newbietrap.