I used the term "significant skill" to refer to something that I don't think an untrained person could do, but that a reasonably well trained performer could (whether on violin or with their voice). Certainly not rocket science. As a reference, it took my wife about a month of twice-weekly singing Bulgarian music to really be able to "hear" the correct tones, which of course were even further from her own musical practice (she had been a professional singer earlier in life) due to not just JI/WT but quarter tones etc.

Good point about the open strings though. Less of an issue on instruments from other cultures like the kamancheh, where you would rarely play an open string, but certainly true on multi-strings where the open string situation is "oft-used".