SSB Goku and Ikari Broly were pretty much even. Broly then went SS. So he is 50x SSB. He then transforms into FP/Legendary SS and tanks a shot from SS Gogeta who felt the need to go all the way to blue to win. Which implies the jump from SS to FPSS Broly was not small. Even if it was as small as 2-4x this would put FPSS Broly as hundreds of times stronger than a SSB and that's probably a low ball. Compared to whatever you think the multiplier from SSB to MUI is.
There's also the fact that Broly was citied as the strongest they had ever fought. So by default, he should be stronger than Jiren and MUI Goku. Adding in his insane durability since he took many blows from SSB Gogeta and his ability to adapt so fast, I'd say it is not out of the realm of possibility he could win. Another thing to consider is that Goku could only maintain MUI for like a minute. Even IF Broly can't handle both, he has shown the ability to take hits from SSB Gogeta who is massively above this Goku and Jiren. They could beat on him, IF Broly isn't just stronger, MUI runs out, Goku gets one-shot, then it comes down to Jiren vs FP Broly. Where Broly would adapt IF needed and win. Given what we know, Broly can easily outlast MUI if he needs to and is likely strong enough he doesn't have to. I'd say Broly probably wins here tbh.