I've lived in both those cities this decade; the results are in some cases similar, but the issues are in fact different. The thing that's similar: both cities are warm enough so that you can reasonably live on the streets year round. Boston's an intensely liberal city and nobody uses it as an example of how liberal cities always have homeless problems.

Similarly, Google "dallas homeless problem" sometime. Warm city, poor coordination of solutions for the homeless -- whoa, homeless populations are rising! But that once more does not fit the narrative, so nobody talks about it in threads like this.

(Houston is doing much better. Houston, unlike Seattle, has put a priority on coordinating efforts to help homeless. HUH.)

That said you're right -- it's not just about taxes. Seattle is low on revenue, but that's only part of why the city has failed to produce a coordinated response to homelessness. Bad organizational skills are not restricted to the right or the left, though.