Logic Pro X, MainStage all seem to start up, but the AU validation tool cannot open the plugins themselves. If there are standalone versions of the same plugins, they run. The AUValTool does not seem to be a new version number, so this is probably a different issue than the changes that came with the upgrade to ElCap, but... This would be an issue for anyone brave enough to try this without a partition to "play around with" that would have no effect on the business of making music.
The Apple AUs *do* work...
The Apple AUs *do* work...