Are countertop dishwashers a real substitute for a built-in?

Not really, according to community consensus. r/LivingAlone, r/TinyHouses, and r/KitchenStack threads describe them as useful in dorms, RVs, and rentals where plumbing isn't an option, but capacity tops out around 6 place settings and drying performance is mixed. For a permanent kitchen, an 18-inch built-in is the better investment.