You can build these under fire with no consequences. I had 2 heavy bolter squads, a apothecary in green cover. The enemy started building a Tarantula Turret, I focused fired on it, didnt do much damage, and it was built then suppressed all my men and had to retreat. I couldnt get behind it because they had their own heavy bolter squads covering it. They then proceeded to creep forward with Turrets covering each other on the VPs.
Tarantula Turrets should be easy to kill while building so you cant just build one in combat under fire, I.E. CoH while building received damage bonus.