I think we are talking about oranges and apples here. Synthesis is well known in what it does and how it maps intermediate nodes. I am not familiar with the internals of yosys but Tim Edwards is probably right. @User
you should check our prefix adder generators https://github.com/tdene/synth_opt_adders
. I don't particularly agree with your comment that synthesis cannot do better. DC and Genus do a very nice job with syntax such as a+b or a*b. Yosys is a completely different matter but a very interesting one research wise.