Medicine is barely evidence based, so it's not surprising that education is not. Research is expensive, and kids are so variable that even evidence-based conclusions wouldn't work for everyone. In any case, there's no money to do true randomized well designed trials on math curricula.
So instead of evidence we get opinion-based fads. In the late Clinton era we got everyday math, which swept into Minnesota in the 00s. Now it's receding elsewhere, but we're still stuck with it. Obviously Everyday Math must work for some, but it hasn't done well with our three.
So, for this summer, we decided to do something different with #3 before she enters middle school. I started with the homeschool site curriculum reviews, but of course many of them are concerned with religious purity as much as education. I didn't see anything there I really liked. I wanted a well done textbook, but Saxon seemed too rigid and dull and Singapore Math too strenuous.
Next I tried places that I thought might do a good job with Math teaching, such as California and Ontario. I decided I liked Ontario's approach to 5th and 6th grade math best. They use the Addison Wesley's "Math Makes Sense" series. (I enjoyed the exercise animation that used hockey puck weights.)
I found used copies of the 5th and 6th grade books on Amazon for a few dollars each, so for about $12 total we have the material for our daughter's summer work.