The issue with ancient practices nowadays is always the same, they all got corrupted in a way or another. Japan resisted on certain extents to buddhism tentacles, it was sadly inevitable, especially because the foundation of this art came from China. So once you corrupt the core the rest can fall apart.
If you are asking for the practice itself, in my opinion, spiritual practices around the world on ancient times we cannot even recall, took shapes and colors according to the culture of that specific place, to blend better with people heritage and soul.
Japan holds a priceless spiritual treasure, in terms of knowledge, since among the other nations it stood against the "corruption" the most. Altough not for long. We can blame the enemy, but people on their own with their ignorance easily destroy the true nature of our history.
So yes, Onmyōdō it was possibly legit, at least on a philosophycal level, but if you find it in our current era I highly doubt there's something worth of your attention. As I said the notions are similar, just with differents terms and colors.
It's like some people claim sushi is different from fish, when sushi is the japanese word for fish.