Are these assortment kits good quality or just cheap filler hardware?

Most budget assortments use zinc-plated carbon steel, which is fine for general indoor and light-duty use. For outdoor, marine, or corrosion-prone applications, look specifically for stainless steel (often labeled 304) or Grade 8 hardened steel. Verified-purchase reviewers across these kits generally rate fit and finish well, but a recurring complaint is that you tend to run out of the most-used sizes first.