The Freeform Solutions website was built in-house originally on Drupal 6. It was upgraded to Drupal 7 and finally upgraded to Backdrop CMS.
We chose Backdrop because it is based on a mature project with a large depth of modules and functionality. And we chose it because we are also providing services of building and maintaining Backdrop sites and upgrading from Drupal 7 to Backdrop. It made sense to eat our own dog food.