The mess will not be the fault of insects, disease or weather but the result of poor choices on the gardener's part. The problem typically begins when the gardener visits the plant store and buys too few of too many varieties and crams them too close together. Often it's a case of six yellow marigolds next to six cream petunias next to six red zinnias with a dozen purple alyssum lined up in a row. There's nothing wrong with planting a variety of different types and colors of annuals. The secret is in doing it carefully, so shapes and colors flow together rather than clash.
Colors are splashiest when a dozen or more of the same variety are planted in a clump.
Make the clump egg-shaped, tailoring the curve to match the depth of the garden. If it's narrow, make the egg skinnier. If the bed is wide, fatten the egg. Scatter the clumps within the garden area. How close plants are placed depends on the variety and effect wanted.