Fortunately it's a Share, not a Sync. This means that each individual and each fact and each source must be looked at individually to be moved between RootsMagic and Ancestry.
Also, by default, Ancestry trees created by an upload from RootsMagic are private and non indexed (searchable).
I experimented with this but was not happy with the sources from Ancestry (I don't want a separate source for each census citation, instead I maintain a separate source for each census event and capture citation details).
It does help integrate search capabilities, which can do nothing but help. I find a combination of RootsMagic hints and WikiTree RootsSearch are quite powerful.