I think the powders and oils serve different purposes and each have their own merit. For examble
1. powders = color and clean. I use henna, indigo, shikaika, and amla powders regularly and I love them and they color, clean and condition my hair in a way that oils just can't do that.
2. oils = seal and lubricate. I have used amla oil and while it is very very good and I still use it, it cannot replace the amla powder for conditioning my hair and the powders cannot seal and lubricate.
I hope it makes sense . . .