Status update: More was broken than we'd hoped but not as many things as we feared. We are still working on platform stability issues but the search index is being steadily rebuilt and (some) image generation issues have been solved. All other issues have been logged (and triaged) the Mercury issue queue, where you can keep an eye on our progress toward solving them. If you have an issue that is not in the queue, please fill out the support request form or email firstname.lastname@example.org.
What we're currently working on: Seeking to knock out as many issues as possible, so not dwelling on any one issue for too long.
Drupal 9+ Mercury Reader users: Eric Neville solved some issues with the reader module. Grab 2.13 asap (Composer should pick it up).
Drupal 7 Mercury Reader users: Michael Sheldon was kind enough to patch a number of issues with the Drupal 7 version of Mercury Reader which caused it to be unhappy with the new system. The new version is available here: https://github.gatech.edu/ICWebTeam/hg_reader-7.x/releases/tag/v2.20