In the last 90 days, there have been 16 CS => SO migrations and 2 have been rejected.1 That migration volume falls just short of my prefered criteria.2
That said, there have been 94 questions closed with the custom reason pointing to Stack Overflow. Since these were not migrated, we can't actually know how many would have been rejected, however. (Migrating them now would very likely require checking that those questions have not already been reposted. The close reasons suggest this is somewhat common.) Without knowing the rejection rate, we can't estimate how many of those questions would have been productively migrated.
Of course the whole reason for establishing a default path is to make migration easier. As long the path of least resistance is the custom close reason, I suspect migrating from CS to SO will be rather rare. That's not a bad outcome, in my opinion. For people new to the network, having your question migrated to a whole 'nother site can be confusing and frustrating. And, of course, new users are the most likely to misunderstand the scope of the sites.
On the other hand, if moderators are willing to handle more migration request flags and try moving a greater percentage of questions, we could revisit this analysis in a few months.
This doesn't match the statistics seen on this site because of wierdness.
And if you don't like my criteria, you should know that other CMs have an even higher bar.