Tolkien would have had at least a pretty good instinct for the feel of the post-Roman world as he was heavily exposed to primary sources... and lived in the turn-of-the-century English countryside, which would have still felt recognizably pre-modern.

Many subsequent authors will have only seen modern or American or no countryside at all and get their impressions of "the middle ages" from a mostly-fictional cultural pastiche of Tolkien (who was really writing a Late Antique world), Dungeons and Dragons, Monty Python, etc. so it's no surprise their worlds are rather implausible.