Competitor and crash survivors view (grey haired view) Primary safety is a good seat and belt. The seat must stay where it is bolted when subjected to big G's. The belt must hold you in the seat.
Next comes hitting hard things. (trees, armco, road furniture etc) The car will deform; its designed to. The cage will help prevent it deforming onto your little safe area, snug in your seat. If your lucky.
Other than that they are a pain. They are tricky to fit properly, prevent easy access/ingress and slow the car down, due to weighing a ton.
They MAY help the handling, but unless your running mega spring rates, I wouldn't think you'd notice.
IMO
ps: having had a few belter racing accidents over the years, i put my money in safety devices coffers. They seem to work at a livable (ha) price