Sure he was beaten, but not by simple SSJ power. He wiped the floor with what, 4 guys at their best? After beating the crap out of them Goku does what he does best, draw energy from other people, and then defeated him for the moment, notice he didn't finish the job even though he put everything they all had into it. He outclassed alot of charecters as an INFANT, planet explodes and he saves himself and his father at less than 6 months. Insane as it is, he was born at power level 10,000, which makes him a Super Saiyan at birth. NOBODY competes with that. If you just look through most Anime sites that take a look at all the villans of the franchise, you'll always see Broli ranked as the most powerful, seeing that there has never been one that can destroy multiple galaxies without breaking a sweat, as Absolute Anime put it.
As far the Fusion tiring, kinda hard to gauge since they really weren't trying, so the limit of their power is almost incalcuable. Again, Broli's only been seen on 1 level and he's disposed of the main charecters with relative ease, at lv. 3? Not even this Fusion stands a chance, not even at 4. There has never been anyone in the DBZ/GT universe that can do what Broli did at Lv1, hell at Lv. 2!
Broli is literally raw power.