Here's another way to put it: The "zero dimensional" model of CO2 is incredibly straightforward. You add CO2, temperature goes up. It's trivial and undeniable, and all by itself tracks fairly well with the observed warming.
Better modeling adds precision, and has been even more accurate, but is difficult for the non-expert to evaluate. It should be easy to trust its track record, but without that positive gut feeling I get from "We've been burning fossil fuels and it would be bizarre if the temperature didn't go up".
So I'm able to base my confidence in the models on a simpler model that I do understand. It's easy to accept that the existing additions (CFCs, methane, ocean currents, land, ice, etc) are valid (especially since they also track the data so well). But SO2 would be a brand-new variable, so I don't have as good an intuition.
I know it's not brand-new. As with CO2, the basic physics of SO2 are well understood, we've observed natural experiments with SO2 emission several times. It's definitely promising.
But I really want to see a full plan in place and become comfortable with it before we begin to rely on it. Because otherwise, having watched people deny that trivial, obvious stuff for ideological reasons, I expect them to seize on geoengineering as "stage 6 denialism": "It's real, it's our fault, it's not good, reducing CO2 would help, it's not too late... but only because somebody will dump SO2 into the atmosphere and so we should start all the coal factories up again".
