and in some cases, after making people wait for years, then USCIS sends an RFE to prove that the marriage is still going on.
In our case having to prove the relationship for the fiance visa, with the first interview at the embassy, preparing hundreds of pages of evidence for the 485 nerve-wracking interview, countless of sleepless nights, thousands of dollars spent, hundred of pages more of evidence for the 751, and 2 years later, still no definitive answer.
at this point not sure if applying for the N400 is better than keeping on waiting.