I'm not sure that's an experiment we'd see unfold due to NIMBYism. (Not commenting on whether that's an appropriate reaction, just that it's inevitable.) So I wonder if within realistic constraints it's still a fairly safe assumption?

the city i'm in has done it up to 4 units. it's pretty recent (less than a year, i think), so i don't expect to see much effect yet

NIMBYs did indeed get involved: you can build up to 4 units in basically any residential zone except for the "rich people" part of town

more cities seem to be doing this sort of thing. hopefully we'll have more/better papers on it in the near future. (and hopefully it works)