The overscan and sync scroll techniques don't actually change the display refresh frequency. They modify the sync freq registers only momentarily. And except for a couple of cases, these changes don't alter the actual sync signals in any way whatsoever.
However, allmost all demos using this technique will work at 50 Hz only. It is possible to do some border removal at 60 Hz, but not nearly as much as at 50 Hz. Software overscan at 60 Hz was hardly ever used, if at all.
As the OP is saying, the whole thing is not an issue when using an Atari monitor (or any other similar RGB monitor for that matter). When using the TV output, you would get a "mixed" signal, NTSC but at 50 Hz. And the result depends completely on the specific TV (or monitor, or capture device). Some TVs would take it without problems, others would display in black and white, some multinorm ones would require you to force NTSC (disabling the AUTO option), and some would probably refuse to display anything at all.