Hello! I use fluently Drupal and I didn't know backdrop (shame on me, i know).
I installed it and I recognized the Drupal environment which sometimes gives me a headache - Symphony is great but you have to be focused !
But more comfortable for novice users (not me) but my clients who are sometimes often scared by too many pimples. Normal...

For now, I'm using the starting theme mentioned in the document to create my base theme. No problem, to install.

I think my next website will be done with Brackdrop. I hope !