Subdoms are okay, but draggar is right IMHO and they are treated by Google as completely separate domains. That's fine, and you can work a little
SEO with it, but I think one rule is never interlink them.
I own a generic like this; engineers .net, so when I first acquired it, I was excited at the possibilty of sundomming it like:
hvac.engineers
electrical.engineers
structural.engineers
etc.
The fact is that each subdom needs to be built like it was a completely separate site. Or at least that's how I read it.