For regular (non Dreamhost PS) servers, there is a 200 simultaneous connection limit, though if you do a search here on the forums I think you'll find that Dreamhost is willing to work with you on that.
There's no formal/hardline CPU limit, though informally if your hosting affects the health and safety of the server and your neighbors you'll be asked to optimize your code or upgrade to PS or a dedicated server. We've seen a few complaints here from people who've had support make some changes to their hosting like turning off bot access when their sites have been found to be killing the server, so we do know that they do watch out for stuff like that.
I consider it a plus because they're looking out for your own welfare and the welfare of the other "well-behaved" people on a server.
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