From a quick glance, it seems greymatter is still under active devolpement. You might have better luck asking about security issues at http://www.greymatterforums.com/
Dreamhost won't have anyhting against you using the software, enless is poses a threat to the server you're hosted on. But then they'll only notice and have an problem if you get hacked.
If there are still security issues with Greymatter, why not consider somehtign like wordpress, that's aviable as a one click install (with easy upgrades) through Dreamhsot.