My setup uses multiple symlinks to maintain a common core codebase instead of the multisite feature. This has certain advantages (i.e. shared codebase, easy core upgrades, site data has its own directory tree, no cross contamination between sites/files, etc.)
Drupal 8.x was working fine with symlinks 3 weeks ago, but when I updated today, it no longer works. The change is in the definition of DRUPAL_ROOT from using getcwd() to using dirname(dirname(__DIR__)).
This commit nicely consolidated all the DRUPAL_ROOT definitions to just the one in bootstrap.inc. However, unlike D7 which uses getcwd() (see constant DRUPAL_ROOT) it changes the definition to use dirname(dirname(__DIR__)).
I can't say how many people this will affect, nor can I say if dirname() has any advantages over getcwd().