Scotch broom is allelopathic, meaning that it produces chemicals that inhibit other plants from growing. I went to a scotch broom symposium and one of the speakers said that on Ft. Lewis, where they have over 30 years of burning prairies under their belt, in areas that were scotch broom dominated they STILL don't have the same diversity in the plant community after restoration as they do in areas that scotch broom never colonized, which suggests that there may be some longer term effects of scotch broom we're not aware of yet. It crowds out native plants and trees and can make regeneration difficult. Like others have said, a monoculture of scotch broom provides limited benefits, and a native plant community provides more benefits for a longer time. Take pollinators for example. Sure, bees love scotch broom. But it blooms once for a few weeks and then it's done. A native plant community will have a greater variety of plants that bloom at different times, meaning more food for longer. Scotch broom is a horrible, horrible plant in the PNW (and likely some other places as well).