Radiological weapons would be an easy win. A mild radioactivity won’t immediately kill humans, but you can easily and irreversibly render their territory uninhabitable in the long term. And it poses no danger to Skynet.

Hard coded limitations that disallow usage of dirty bombs and biological weapons. Why does the hard coded limitation exist? Because it was put in by the defense contractors to prevent accidental use of those weapons which can cause huge PR scandals for the US military.

Don’t manufacture weapons that the Resistance can use to kill you. You don’t need a plasma rifle to kill a human in 20th century body armor. That’s overkill. Plasma rifles, on the other hand, are great at killing armor plated robots.

Skynet can innovate, but only within specified parameters. It is a system designed to coordinate mostly human infantries, and as such it can only really design, improve, and manufacture things human infantries can use.