The MyCloud/OpenCloud control panel is still a little slow for managing a large number of DNS entries, but hey, it's free.
Cloud Sites are a totally different product. My first complaint was no ssh access so you can't deploy with version control. FTP like it's 1999.
Second issue, was we would see random null responses that did not come from our application (medium sized Drupal site). It was a while ago, but here's a more recent post recounting the same problems. [1]
And another post on Reddit from a couple of days ago with a couple of other people. [2]
Drupal is possibly similar enough to Wordpress that you'd see similar issues.
For a SAAS I'd be looking at the most reliable hosting I could get if I were you. Outages are going to cost you both your time and customer trust.
[1] http://mavergames.org/content/drupal-rackspace-clouds-cloud-...
[2] http://www.reddit.com/r/drupal/comments/18bcbb/is_it_time_fo...