Can you elaborate? Putting aside rowhammer-like vulnerabilities [that don't fall under "overflows" anyway], how is shared physical memory under a modern protected virtual memory scheme leading to overflows? Additionally, the word "buffer overflow" usually refers to overflows that take place in the same process.

The process crosstalk (information disclosure, heap grooming, heap manipulation) that takes place to trigger the overflow, wouldn't be affected in the least by physical separation. In many cases (exploitation over the network), you have exactly that.