They are up to a point. Once you start hitting/getting hit by bases with Colossi in them these die very fast if you aren't careful. However, as a mixed force with Colossi they can work really well; try to empty the bunkers and at least one missle turret first then send these in to take out mortars, or at least as many turrets as they can before they fall.
Well, a level 4 starbase is quite low level; using these is a bit of cost overkill but yes they would be quite effective. Higher than that where you start running into defense bunkers with the Colossus, these can really get into trouble.
What do you think?