Correct, it is a bad idea. Each hashing blade pulls ~460w, which would push a 900w psu over its rated limit. While it would work for a while because server psus are pretty beefy, running hardware out of spec for that long will eventually cause a failure of some sort.

If you could find a 1200 watt version you could run 2 of the blades off of the 1200, and the third blade + controller off of the 900w psu safely.