Mixed tableware beats a matching set every time, because a matching set is only ever as good as its weakest piece, and the day someone chips the gravy boat the whole set starts looking like a museum display with a gap in it. Buying plates, bowls and serving pieces separately, chosen for how they feel and stack and survive, gets you a table that looks assembled by a person rather than issued by a catalog.
Where it goes wrong is scale and weight. Thin, pretty bowls that look wonderful on a shelf can feel flimsy in the hand and chip if a fork so much as looks at them wrong, so pick one up in the shop, knock a knuckle against the rim, and see if it answers back with a dull thud or a proper ring. A dull thud usually means it's going to survive a dishwasher cycle and a toddler's grip, a ring means handle with care forever. This kind of open, mix and match approach to tableware suits a kitchen that gets used daily, a rented place where you're not trying to impress anyone, a family table with three different sizes of human eating off it at once. It struggles in a house that does a lot of formal entertaining, where guests notice if the side plates don't match and start wondering what else you've let slide. For color, plain white or a soft cream earns its keep because it goes under any food and next to any other piece you add later, and a warm stoneware glaze in rust, sage or a dirty blue hides the small scratches that build up over a few years of real use rather than showing every one of them under a kitchen light. Stay away from anything with a heavy metallic rim if you're putting it in the microwave, that's a lesson you only need to learn once, usually with sparks involved. It clashes hardest with itself when you go too far the other way and buy a different pattern for every single piece, because then it stops looking collected and starts looking like a church sale. Two or three finishes in the rotation is plenty. It also sits oddly next to very formal, gold rimmed glassware, the two styles just argue with each other on the same table and neither wins.
Get the weight and the glaze right and even a plain weeknight plate makes the table you already own look like somewhere you actually chose to sit down.