Right - this is another tactic that's impossible to deliver 'unlimited' of, but hosts will oversell to the point that it's still cost-effective for them, before giving you the boot. From our tests and experiences through dedicated customers, a good paid hosting server can handle a few thousand domains before it explodes from the all of the automated processes each domain has (stats generation, dns sync, automated backups, etc.), although you'd probably start noticing slowing response times after a few hundred.
Of course if for whatever reason you had 600 domains on a $2 unlimited-domain plan, you'd probably want a cluster of a few servers to keep respectable performance - which I guarantee you, these companies are not supplying. But for a few domains, you may very well still be happy with one of these companies - assuming they don't attempt to honor the 600 domain customers by shoving them on one server with you. Just be sure you understand what's going on here.