Do you have the 'Remove from "changed" list after accepting changes' box unticked? (File Options / Ancestry)
In theory if you tick that then the person remains on the 'Only show changed people' list so you can check if there are other changes. The exceptions to that seems to be people who you add as children or parents of the 'changed' person. Thus, if I add a mother and/or father of someone whilst amending the person, some of their details are not passed over and the parents disappear from the "changed" list. I then untick the 'Only show changed people' box and find the parents to complete their marriage details. I try not to add several generations of a family at one time as I can easily forget to do that.
There doesn't seem to be any easy way of finding the people with details on RootsMagic that are different from Ancestry once you have gone through the TreeShare process. I don't just use Ancestry for data but I tend to add items to my Ancestry tree and then use TreeShare to add it to RM. It almost makes more sense for me to download a new tree from Ancestry via TreeShare every so often rather than periodically updating RM.
I like having a backup of my tree and I use RM for creating many reports so I wouldn't want to take RootsMagic out of the equation.