(Have done this a few times. Was hesitant initially because of the low price and used nature, but when the first stick passed MemTest86+ including RH perfectly, I was convinced. Half a dozen more sticks later and still good. But maybe they've started to run out of the good old stuff now...)
That's demonstrably false. I remember rowhammer being thought of and demonstrated on DDR3 modules.
The tl;dr is that everything 2009 and older is good, 2010-2011 is possibly bad, 2012 and newer is almost certainly bad.
Software that attempts a hostile rowhammer attack aims to be unsafe by definition. It's not a matter of trying and failing to be safe. Rather, we're seeing that it's very difficult to build hardware and software that prevents hostile code from running amuck: if the attacker can run hostile code on your computer at all, you have already lost. That's bad news for cloud hosts and confirms what we already knew about web scripting, but doesn't seem all THAT bad for the rest of us.
Of course it's possible to build computers with no DRAM at all (i.e. only SRAM, which costs more). That is not economically sane for big servers, but might be the right thing to do for security sensitive components.
Yes, and this is actually the problem. Without safe hardware, it is almost impossible to write safe software.