The wiki page has a pretty detailed list of breakages, for eg "dpkg-query -S is currently broken by this approach". Hopefully the in-progress patch for some of these issues will get included.
I don't believe the list is detailed enough, because it just says "thing is broken", but not under what circumstances.
As best as I can tell, `dpkg-query -S` is broken by this iff it's passed a path to a file that's been installed under a different version of that path.
E.g. `dpkg-query -S /usr/bin/vim` fails if vim was installed via `/bin/vim`.
That's a minor bug, that should simply be fixed in dpkg, and that's also easy enough to workaround if the distribution simply installs all files in /usr/bin via /usr/bin.
None of any of that seems to be enough to unilaterally hold up a distribution-wide decision to move to a merged-usr, especially not via official sounding warnings in the install script for a major distribution component, and especially not when this way of doing things works without a lot of complaints in other distributions including the related Ubuntu, and especially not to call for a special Debian solution that has its own problems and to do so years after the fact.
Frankly if I was a debian developer I'd be quite cross with the dpkg maintainer.
What exactly is "behind dpkg's back" here? This was discussed, in the open, years ago!
This was implemented, as an option, years ago. This was implemented fully in other distributions years ago! Fedora has had it for a decade, with few problems.
Dpkg has a few minor bugs with it so it needs to be fixed. It's holding up progress here.
In that usrmerge adds symlinks that dpkg does not know about, doesn't manage and doesn't understand. Like if the sysadmin added random symlinks in various places. All bets are off after that. I'm surprised the amount of breakage isn't higher TBH.
