I avoided matching frame sets for years because every one I saw in a shop looked like it had been chosen by whoever did the lighting in a dentist's office, same wood tone, same mat size, marched up a wall in a dead straight line. What changed my mind was seeing a set broken up and used loosely on a long console table, mismatched photos, gaps left on purpose, and realizing the "set" part was really just a shortcut to buying frames that already agree with each other, not a demand that you hang them like a chart.
Where they go wrong is in rooms that already have a lot of pattern or a strong wood tone doing its own thing, a busy wallpaper or a dark walnut sideboard will fight a set of black metal frames for attention, and neither wins. Get the scale right before anything else: measure the actual wall space or shelf run you have, lay the frames out on the floor in the shapes you're considering, and only then commit, because the single most common mistake is buying a set sized for a showroom wall and ending up with five frames crammed onto a narrow hallway strip, or three tiny ones lost on a wall over a bed. A good rule is to let the whole arrangement take up somewhere between a third and two thirds of the furniture or wall section below it, not the entire thing, and not some lonely little island in the middle. If the set comes with mixed sizes, resist the urge to use every single one just because it was in the box, sometimes four frames read better than seven.
Check materials with your hand, not just your eyes. Painted wood frames feel warm and take knocks without showing it much, but cheap ones chip at the corners fast. Metal frames, especially thin gold or brushed brass ones, scratch more easily than they look like they should, and a hairline scratch under a shop light is invisible until it catches afternoon sun at home. Open the backs before you buy, the little clips and stands matter more than people admit, flimsy ones bend the first time you swap a photo, and a frame that won't stand up straight on its own is a frame you'll be adjusting every week. Glass versus the plastic look alike matters too, real glass sits flatter against a photo and doesn't yellow, the cheaper stuff can warp slightly over a year or two.
Over time the wood ones mellow and pick up small dents that honestly make them look more lived in rather than worn out, while painted metal frames tend to lose a little shine at the edges where hands reach in to adjust a picture, which just ends up being its own kind of record of how often that shelf gets touched.