Gods are generally not accepted. Angra Mainyu was a human originally, and only took that form because of his village's belief. The form summoned in the Grail War was incredibly weak because he was just a human, with no actual combat skills.
Everyone knows about Frankenstein's monster, so at this point whether it existed or not is irrelevant.
Someone living in the 20th century would probably be fine if they're well known, but the rule of the series is the older the stronger so anything you summon from the 20th century is probably gonna be pretty weak.