The Fetts have demonstrated excellent endurance (surviving falls and explosions, Boba surviving against the Sarlacc's stomach acid), strength (kicking down durasteel doors, carrying D'harhan), and arsenals (poison darts, flamethrowers, whip-cords, mini-concussion rockets, sonic emitters). No doubt the father and son duo would have great synergy too. Against a mid-level jedi or sith, their victory may be assured. But they are fighting Grievous, and not TCW Grievous, but peak Grievous. A Grievous who has been stated to be able to withstand starfighter laser cannon fire, a Grievous who can crawl along walls like a spider and who can do so at a speed where he can dodge blaster fire from ARC troopers (who possess Jango’s genetic template and inherit his marksmanship) and who can run fast enough to evade a barrage from a Republic Gunship. Boba can crush rocks in his fist, Grievous can crush skulls. Jango and Boba’s rockets can bring down stone buildings, but Grievous has survived being in the heart of an explosion that threatened the stability of a planet’s surface. If the Duo split, they risk hitting each other with their AoE weapons that will likely do more damage to their partners than Grievous himself. Grievous has killed Death Watch commandoes, armed only with his claws, and has shown that he can stab right through their armor with his light sabers too. Grievous still takes this imo, though the discussion of this topic has probably run dry by now, I just wanted to state final thoughts.