Since each of the services (Ancestry, FamilySearch and MyHeritage) are separate entities, some for profit, some not, the issue is that they do not work and play well with each other.
Also, they have overlapping records (manifests, census, marriage, birth death indexes), use different sources of the same material (marriage indexes that may or may not list parent's names). Some records are transcribed, some are not.
I found most of my family's manifests through Steve Morse because the search is much more flexible and refined.
An additional issue is caused by affiliate sites (FamilySearch & BillionGraves, Ancestry & Find-a-Grave) which have their own content, acquired independently from each other.
And there are sites that require additional membership (Newspapers.com, Fold3) that are integrated into the Ancestry results.
There is no easy way to manage integration if you have trees on multiple sites, or work with family members if they have separate trees.
On Ancestry, there is an option to add a web link for sources that are online but outside of Ancestry.
The downside is that they don't link to an event in the Facts list or appear in the Gallery or Life Story.
You can add a new event, which will appear in the Facts list and Life Story and add the link in the description but it isn't clickable.
I don't know if FamilySearch or MyHeritage trees have that option, I suspect they have something similar.
San Francisco, CA