Lack of violence
"The bipedal locomotion we're familiar with is something humans have evolved over a few hundred thousand years and it turns out it's surprisingly tricky to replicate."
This is somewhat missing the point, I think, as the tricky bit would be to have a set-up being able to adapt and evolve to that degree at all, happening gradual or accelerated artificially. Because it's not like legs have evolved in a few hundred thousand years, or the nervous systems and muscular system enabling the adaptation to specialize into, for example, bipedal locomotion. The amazing thing is the underlying "machinery" being able to develop in so many directions, when stressed to do so.
Trying to replicate this one bit will not easily lead to getting anywhere. One has to understand why the underlying basis is so damn good. Probably a lot of violence involved though, as was with the Nexus 6... which might also explain why it's a refined, basically still 19th century device and not a killer replicant on your desk.