So I was wondering something: If I wanted to create a homebrew setting (Greenbone series by Fonda Lee) that was an Asian Modern/Urban Fantasy-themed but not tied down to the specific L5R/Rokugan setting. How useful is the L5R material from a crunchy rules standpoint?

Specifically, I'm mostly interested in Asian-themed weaponry which L5R has in droves. To a lesser degree, I'm interested in their takes on Magic/Kiho and the Monsters.

Short answer: Not very.

Longer answer: The two games have very different systems. While FFG's L5R was obviously influenced by some aspects of Genesys, nothing's really 1-to-1, and would help about as much as any other system would.

But I can link you this for your consideration. I'm sure people over on the Genesys forum would be more than happy to aid in any advice for making what you want for Genesys as well.

The book of fire in the L5R conversion is basically what I was looking for, thank you.