Sugihara deals with opponents quickly and efficiently, whereas Kirishima inflicts PTSD.
Kirishima is probably more fearsome than even the boss. The boss may be strong, but he's not that wild.
That's why the boss entrusts Yaeka to him - the safest place for her is beside the ultimate badass.
That, and Kirishima's tendency to go overboard makes him less suited to normal mob work.
(It doesn't help if you scare off the people you're trying to extort, or render them incapable of earning those funds.)
Looks like there's the added benefit of taming him down a bit too; but that doesn't mean he's not still scary AF even after cooling off a bit.