I use the tabs to hold the thing in place after aligning it to my marks. Then I use 1 1/4" drywall screws at all four points on drywall - techs on metal.
You can use nails if you like, but I do a lot of custom work where the client (unfortunately) sometimes wants to deviate from the plans and make changes. Nails become a nightmare at that point.
I would never recommend using just the holder-tabs to secure a fixture. They are completely insufficient and tend to pop right out when you slide the fixture into place for final adjustment.
