Hello Savinja. 
I understand your comment about the outlines of buildings. When you gain experience, you will recognize buildings better. :-)
About the Imagery.  At the start of this project. Maxar Premium was the recommended Imagery. (for many projects all over the world)
A time ago The Maxar Premium imagery was made unavailable for us ( safety issues ??) .
Since then, we fall back to other imageries, here Bing, with parts showing older areas.  So it is possible that mappings with Bing as imagery show no buildings where a previous mapper could find and map them on Maxar P. 
And then we have an offset issue as well.  Don't change to much.
We try to deal with it.
Best regards