UW is a weird school, so it's hard to read into their match list too much. First, as a class composite, there is heavy selection for primary care, rural, and underserved focused students. Academic requirements for admission are less stringent as a result (think 3.0 GPAs and 500 MCAT scores), and many students are perfectly happy with primary care or lower competitive specialties. Second, a large portion of the school (probably the majority) grew up in areas with a total population under 250k. Some of them have no interest in going to a larger city for residency.
Out of the students who try to match well, the reputation of the school usually carries them well enough. I've seen statistics that for any given specialty, the average step 1 score for students who fail to match their preferred specialty are about 10-15 points lower than national average (implying that some students with low scores still match, probably because of the school's reputation).
This year, there were 28 prelim matches (you can assume about 15-20 of those were for categorical PGY-2 prelim years). That means anywhere from 3-5 percent matched only prelim years (which isn't great, but isn't terrible given the class composition.)
With all that being said, I know students who failed to match neurosurg, ortho, vascular, ENT etc. with relatively decent CV's over the last couple years. I have no idea what percentage this is in total. There are certainly parts of UW that are parallel to top medical schools. One of the big problems are that many of the residency programs are too competitive for home students, which can make matching difficult for an average student. There also isn't a ton of loyalty to home students.
It's almost like there are two distinct schools at UWSOM. There is the local, rural, primary care, underserved, and state school side that is the main reason many students chose UW. Then there is the academic, specialty-focused cohort of students that chose UW because of its academic resources and reputation. In general, the school is able to provide adequate resources for both types of students.