My site is now running on Drupal 9! The upgrade was pretty straightforward with only a few things to correct in some custom modules so they could be read and registered in Drupal 9. On deck are a half dozen client projects to bring from 8.6+ to Drupal 9. I don't expect a lot of issues there, either, despite those projects being an order of magnitude larger and functionally complex than mine. It's not about scale - it's about process.
This is the first Drupal major version upgrade where I didn't have to throw out / convert modules, or spend a few hours debugging "why" an error is happening, or even migrate previous data. That's amazing. The hardest part I ran into honestly was installing and swapping PHP 7.2 for 7.3 on my AWS instance (7.3 is the minimum requirement for Drupal 9). But otherwise, very cool.
I wrote two articles recently for Velir. One of them was about Drupal 9 release, and the section with the migration path proved to be true. It's incredible how the community rallied around the contributed projects in the last 12 months to get them ready for a smooth release.
The other speaks about what a client should look for and questions to ask of an agency to select the best fit partner for them. Given the events of 2020, it will be critical for organizations to review and evaluate if their current vendor is up to the task of supporting their online endeavors, the article goes in depth of what makes a great partner.