In my opinion you have come to the right place
DreamHost is a very good choice for those intending to host multiple sites under the one account. For all intents and purposes these sites can be kept independant, each having its own public root directory etc. The sites can be under the control of one machine/ftp user, or you can choose to place each site under a separate user.
About the only meaningful restriction you will have to consider is the CPU usage limit, with each account limited to 60 CPU minutes per day. In reality, you are unlikely to run into this limit unless your site(s) are extremely busy and/or you are running some dodgy un-optimised scripts, forum plugins seem to be a major culprit.
I can't give you an exact figure on uptime, but since hosting with DreamHost I have only noticed a few very short outages. Once DreamHost is notified (via the Web Admin Panel), things are usually rectified very quickly. In fact, on a couple of occasions the outage was rectified before I had a chance to notify them.
Of-course, I can only comment on the uptime of my particular server ('Bixel'), DreamHost has many hundreds of servers, so the uptime of them may be different.
The problems reported after the recent plan changes were initially thought to be caused by the influx of new users overloading available resources.
This turned out not to be the case. A DreamHost staff member reported that the problems were caused by an internal router that was not performing as expected. The router was replaced and (as far as I know) the problems went away.
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