I exported MH tree as gedcom that I imported in RM. I had plenty of duplicates (when you use smart matches to import new family members, MH does not prompt you there is already someone with that name in the tree), capitalisation errors, etc.
The in-build tools of RM did a really good job to have a consistent input but merging people can double facts, parents,... though. So a lot of manual checks (List of facts that appear more than once per individual, count trees, etc) were also needed.
Then I used many queries to clean and have a clear view of what is reliable and what is not. For example, I add the symbol * in the suffix of everyone to show that the person is from a shared tree (unreliable, need to be checked with official sourced) and put 'about' in front of all the dates (I take it out when I find a reliable source).