Seems to work well for some buildings sometimes but not at all for others.
eg: my last game I set the shield, troop defense and clone facilities to be built at 50 million pop. well before I got the techs for them, when I got the tech for the troop def. facility they all built automatically, the Planetary Shield seemed to activate at 200 mil. (in 2 instances I watched) and I didn't see the Clone get auto-built at all.