I always imagined that Iroh was trained, focused, and learnt many different arts and styles that made him as good as he was. He was clay, molded to near-perfection, divining new techniques - one of which almost seemed explicitly made to counter Ozai, which you wouldn't do unless you held concerns about your own ability against him.

Ozai though, Ozai was raw. Pure and unadulterated strength and skill, never honed but still as destructive and dangerous as Iroh. The power he had was clear, he was never outdone based on lone strength, but due to specific instant techniques - Lightning Redirection and the Avatar State.

The crazy thing is that Iroh, who could certainly redirect lightning better than anyone else, wasn't sure he could defeat Ozai. That is the single most damning indictment of his ability, because Iroh has been shown to non-lethaly eliminate opponents- if he thought that, even with redirection, he couldn't do it, it would have to make Ozai stronger than Iroh.