I hope this isn't a subjective question. I want to know what Google Scholar lacks compared to paid services that lawyers use so I know what I am missing before I go and try to have an argument with a lawyer.
Google Scholar is not specifically a legal research tool. The search does return many court and legal documents
Which court opinions do you include?
Currently, Google Scholar allows you to search and read published opinions of US state appellate and supreme court cases since 1950, US federal district, appellate, tax and bankruptcy courts since 1923 and US Supreme Court cases since 1791. In addition, it includes citations for cases cited by indexed opinions or journal articles which allows you to find influential cases (usually older or international) which are not yet online or publicly available. From https://scholar.google.com/intl/en/scholar/help.html#coverage
but by reading those inclusions, you can see that it's not as complete as some of the paid commercial services available.
The most important criticism of Google Scholar for legal research appears to be the lack of a way to fully Shepardize cases; see Shepard's Citations - Wikipedia
Shepardizing determines if cases have been overruled (or reaffirmed, questioned, or cited by later cases). Google Scholar will show case citations; but they are not as complete as commercial services. See https://www.google.com/search?q=google+scholar+shepardize for critical references.
If you're going to argue with a lawyer, completely Shepardizing your relevant cases is rather important.