Do shared cores add any attack vectors other shared machines (with dedicated cores) don't have?
The interesting question comes when people start implementing hardware hypervisors and what is the risk profile there.
Security, at the end of the day, isn't about what is secure and what isn't. If you want to be secure, don't get on the internet. Everything else is a exercise in risk tradeoffs and mitigation.
If I was doing anything with PII, cc#'s or any other data I never want to touch I wouldn't use shared hardware without hard thought on it.
For example, they'll typically be on secret mailing lists and aware of security vulnerabilities weeks before you know about them.