Of course, there are many caveats to comparing disengagement rates. These are self-reported by companies, and each company may have different policies about which disengagements are serious enough to report. Also, not all miles are equal. Cruise likes to emphasize that its SF miles are more difficult than Waymo's more suburban Mountain View miles.
Here's a nice summary of the 2017 data I found for you: https://thelastdriverlicenseholder.com/2018/02/01/disengagem...
If it didn't work this way you would be incentivizing operators to train their drivers to take risks in order to hit disengagement metrics which would be bad.
I'm sure unprotected (i.e. no arrow) left hand turns remain one of the more difficult routine scenarios to fine tune. What works in (I assume) Chandler Arizona and San Francisco are probably two very different things. A large busy city like San Francisco simply requires a degree of aggression--Pittsburgh lefts, cutting through fairly small gaps, etc.--that would be inappropriate in a calmer and less busy environment.
Note: I've already got a negative opinion of Cruise regardless of their purported abilities. I followed up on some headhunter spam from Cruise a year or so ago. I bailed before even getting to a phone screen because as persistent as they were, they bailed or were late every time I agreed to one of their proposed times.