At LSSJ, Broly’s Chi continued to grow over time, as opposed to SSJ3 where the Saiyan loses Chi over time, this is a distinct advantage over every other super saiyan form.
It’s written in the Saiyan genome, every thousand years a new Saiyan will be born with the capability of becoming the most powerful form of Saiyan, LSSJ.
LSSJ>SSJ3.
The reason Broly was so weakened after Goku hit him was because he was hit where a blade had pieced his rib cage as an infant.
And once he went back into base from LSSJ, the most powerful Saiyan form, he lost it, his strength and the specific powers of LSSJ.
When he tried to go LSSJ again in Second Coming, his hair wasn’t green, because he had became an amalgam of LSSJ and Super Saiyan one, greatly reducing his LSSJ powers.
As an LSSJ, over time he would have became more powerful than Super Vegetto, if he stayed in LSSJ form constantly.
This quote shows that over time Broly will become more powerful than Super Vegetto.