"Are ANY of these "sources" (Millennium File, Us and International Marriage Records, Family Data Collection, etc) reliable?"

Absolutely not. They are mostly compiled from submitted Family Group Sheets, and to some extent from published genealogies which might be sources of the also-included FGSs. The genealogies may have some researched truth, but sadly a huge number were based lore and surmise only. Beats me how someone who worked every day for 30 years a 5-minute walk from the Courthouse, could not take a few lunch hours over the decades to look at the easily-found documentation disproving some of what he published. Took me just 10 minutes (waiting for the copies took longer).

The FGSs might be compiled from the LDS's IGI, which in turn was mostly compiled from yet other FGS's, which were mostly full of speculation and surmise. This process was a forerunner of mindless tree-clicking.