Memtest86's rowhammer test is pretty reliable. In fact, since the test's inclusion, so many Rowhammer-related errors have been observed that the developers of Memtest86 split the test into two passes (a high-frequency hammer test and a lower-frequency, rate-limited hammer test) due to the sheer number of users failing the high-frequency Rowhammer pass. [1]

[1] http://www.memtest86.com/troubleshooting.htm#hammer

I wonder what is the return rate for memory due to this test.

I guess the number of "enthusiasts" running Memtest on their (or their acquaintances') PCs still is miniscule. So in my opinion no one would notice, except if a return questionnaire explicitly included a

    [√] error while using memtest

field. Keep in mind that newer versions of Microsoft's Windows include their own memory tester. Most people will, if at all, use this.