Oregon passed legislation very similar to this in 2019. It probably hasn't been long enough to determine whether or not it has been worth it, but I don't really see the downsides. Yes, it could make traffic worse, parking worse, etc... but the idea is that you eventually reach a density where that is less important as transit becomes more feasible. They more or less needed to enact this since they have very strict land use laws and the urban growth boundary system (not a bad thing, but if you can't build out, you must build up).

As for applying it to the whole state, I think that's good as well. There are plenty of smaller communities that are in dire need of housing. Being able to build duplexes, triplexes and quadplexes is a good thing since it is impractical to buy several houses and try to put up a larger apartment complex. There is reason to believe there's less pushback from neighbors when we pursue infill projects like this.

Banning single family exclusive zoning doesn't mean single family homes don't get built, it just means landowners are free to pursue building more dense units per plot. It won't be without its problems, but housing costs are not sustainable now and it won't get better until we can build more. On balance I think it will be a positive for the state.

Oregon SFH Ban