Files that were previously in /usr/bin or in /bin can now be found in EITHER of these locations, since one symlinks the other. So no previous expectation was really broken.
Only software built on merged systems fails to run on unðmerged systems. This should not really happen, since the usr merge was a one way trip, not a flag that you're supposed to turn on and off. You'd also never build dynamically linked binaries for BSD on Linux, so that should not be an issue.
But, for some reason, Debian chose to make this merge something that individual systems could turn on and off, which is a terrible idea for part of the base system. It's like letting users pick if they way `/bin` or `/binaries`. Having such heterogenous setups in regards to something so basic and foundational is asking for breakage.