SSJ4 is a non-canon transformation (complete though). SSJ1, SSJ2, SSJ3, LSSJ, SSJ4 are complete transformations, USSJ and USSJ2 are not.
Yeah, Broly always gets defeated by a strong attack of multiple people at once. But it's clear that he's stronger than trained or untrained adult SSJ2 Gohan (please don't argue about SSJ2, it's confirmed in Wiki plus it makes sense if you think about it).
It's always been a mystic thing about how really Broly got defeated in "Broly: The Legendary Super Saiyan". I guess one super strong punch that contains like 5x full power SSJs' power ir enough to knock him down, but not killing him though. (Not Super) Perfect Cell should also have trouble with that kind of punch. Or maybe he wasn't expecting for Goku to really hit him and that's why he failed.
I think that Broly in "Broly: Second Coming" was defeated by SSJ1 Gohan, SSJ1 Goten and SSJ1 Goku's Kamehameha since I highly doubt that Broly could get killed by 2 SSJ1s.
Bio-Broly isn't real Broly, he's just some creature that contains Broly's DNA. Yet he fails.
I never was a real Broly fan, but IMO he really is something. LOL, he can even survive in cosmos, like Frieza. And that unique Legendary transformation...
LSSJ3 Broly must be somewhere between Gohan Buu and Vegetto.