Broly was born with a "Battle Power" that was completely unheard of, so high that he beat even Vegeta's battle power. In the original movie, it was 10,000. But in the new movie, by the time he was 4-5 his power was at 920. Let's apply that to highschool DxD.

Recently exiled by Vegeta the 3rd, Broly is sent off to a backwater planet in the far reaches of the Galaxy, a planet called earth.(Taking some creative liberties) Upon landing on the planet, Broly leaves his pod, confused by his surroundings. Night falls and the full moon rises. Broly wanders around the surrounding area, looking for anything to survive off of. A couple of stray devil's come out of the shadows, looking for a tasty morsel to eat. They notice the young Saiyan and one of them smirks. "Oh, what's this? Did a monkey youkai get lost?" It taunts. Broly tilts his head in confusion, glaring at the stray devil's. He tenses as he senses danger coming from the two beings. By chance, he catches a quick glance at the moon, and he freezes. His breathing quickens as he grows in size, and fur starts covering his body. The two devil's start cowering in fear as Broly transforms into a Great Ape. With a terrifying roar, Broly sends the strays running for their life, but the Great Ape releases a ki blast from its mouth, which kills the strays cooks the flesh on their bones. While rampaging, Broly encounters several different races of supernatural beings, some escape and some don't. By morning, Broly transforms back and lays weak from his transformation. Soon, a woman appears. A woman with blonde hair and 9 Fox tails. She takes the boy in, and brings him to Kyoto. While unconscious, the Zenkai boost starts taking affect, strengthening him as he heals from the strain that transforming placed on his body.

There, with that short bit, I just explained 2 of the reasons that Saiyans are broken when it comes to most anime.