Harald,
I want to keep my Ancestry tree for blood relationships only. No in-laws except spouses, and my relatives only, of course. I'm willing to pay the price of maintaining both trees.
Sometimes the MyHeritage matching service finds new people for me. I have found even more blood relatives there. It allows you to look in on the other trees and to import their data into your own MyHeritage tree. Still, importing data into Geni is a manual thing. Now I feel I must maintain three trees to keep current with crowd sourcing.
AFAIK, Ancestry will import a gedcom file to build a new tree but will not update an existing Ancestry tree. Finally, have you ever tried to create a gedcom file of blood relatives only in Geni? Unless they've fixed it since I tried, the file includes a lot of non-blood profiles. My MyHeritage tree is blood only, so I could generate a gedcom there, but I still can't use it to update Ancestry.
I could delete and completely replace my Ancestry tree, but I'm sure you know why that's not a good idea.
BTW, MyHeritage recently discovered some new people for me in FamilySearch trees. Sigh.