Radiation driving in spectral lines not only boosts the radiation force, it also solves the problem of the star potentially blowing itself apart:
As the line-driven material starts to move off the surface of the star, it is Doppler-shifted, making a previously narrow line broader, and increasing its ability to absorb light.
The Doppler desaturation of optically thick (opaque) lines allows a hot star wind to bootstrap itself into existence!
And causes the radiation force to deviate from strictly 1/R 2 behavior: the radiation force on lines can be less than gravity inside the star but more than gravity above the star’s surface.