I mean, Bege is also part of Big Mom's crew. I don't think that matters much besides, Big Mom herself wasn't fully convinced.
True, Cracker had the abilities to stall out G4's timelimit and eventually beat an exhausted Luffy. Despite G4 two shotting the biscuits.
But that doesn't really mean Luffy was weaker than YC3s or anything, especially in G4 forms.
Reminder that Luffy was matching Big Mom's physical strength & CoA with G4's punches, Boundman is incredibly strong raw force wise. It was overwhelming Doffy & Kata when they tried to face it physically and was sending them flying 100s of metres.
Because that amount of explosive power is timelimited, how G4 Luffy performs is entirely matchup dependent. All the villains who fought it where tailor made to stall it - Doffy stalled it with his aerial mobility & building sized strings with townwide AOE, Cracker had his biscuits & Kata had his FS/Intangibility combo. It would fail to beat characters who are tricky and have abilities suited to stall like them but would absolutely wipe the floor with characters who have no choice but to face Boundman physically like Jack or Smoothie. No surpise though, because G4 was made to fight Zoans and absolutely overwhelm them physically.
Katakuri even noted that when he admitted that G4 was a superior force to Cracker.
Then again, Doffy isn't Luffy. He can easily deal with Cracker's biscuit spam by using his own awakening to turn the biscuits into strings. Katakuri demonstrated that when he turned Mirrorworld, Brulee's DF ability into Mochi.
And unlike Luffy, Doffy isn't limited by time as well.