The main problems with roses locally are not related to climate, soil, insects or disease. Warm and semi-arid climates of California happen to be some of the best places in the World for roses. Sure, many roses have problems with insects such as aphid, and diseases such as powdery mildew, but primarily because such pathogens proliferate among roses that are not pruned properly.
Yes, the main problems with roses are a direct result of improper pruning. Without adequate pruning, roses become overgrown thickets that shelter the pathogens that afflict them, but also lack the vigor to be resistant to damage. Like so many other domesticated plants, they were bred for maximum production of unnaturally big flowers, at the expense of natural resistance to pathogens.
Pruning eliminates superfluous growth and improves air circulation, which interferes with the proliferation of most types of pathogens. Most pathogens overwinter in fallen foliage that should get removed in the process. Pruning also concentrates growth of the next season into fewer new stems, which stimulates vigorous growth that hopefully grows faster than the pathogens that infest it.
Roses should be pruned while dormant in winter, after defoliation, and before buds start to swell at the end of winter. Hybrid tea, floribunda and grandiflora roses should be pruned back to only about three to six canes that grew from the base during the previous year. Older canes should be removed. Remaining canes should be only about two feet tall, and cut just above a healthy bud. Any growth below the graft union (where the basal canes originate) are genetically different suckers (from the understock or rootstock) that must be removed.
As growth resumes in spring, well pruned plants will produce fewer stems and blooms that are significantly more vigorous than those of inadequately pruned plants. Overgrown and inadequately pruned plants must spread their resources thin amongst more but significantly weaker stem growth that is much more likely to be damaged by pathogens. Aggressive pruning now pays off later.