Hypothetically speaking, how useful would something like that be in a nuclear weapon fallout scenario? Can such a contraption detect the important isotopes and give the user an accurate idea of the level of danger they're in, does that change over time as isotope composition changes through decay?

Are you willing to bet your life on it?

tl;dr not useful. To make a long story short, max range is very important and these cheap tubes saturate easily, and they don't give energy-corrected dose rate. They also cannot do gamma spec (isotope identification), but that is not needed for a fallout scenario because fallout contains basically every isotope under the sun, no need to do any gamma spec. I have a very long detailed video about this topic on my youtube channel BetterGeiger