Last year we used MFS alot because of unique Covid rules and situations.  That saved thousands of dollars in most cases.  I used to do it for couples where one had high income and the other just average income.  Child tax credit limit used to be alot lower so you could gain quite a bit by lower income spouse claiming kids.  Now limit jumped up to $400,000 so it not so popular.

Like Ohio, in Iowa we file 85% of clients separate on Iowa return even if joint on federal.