The "hard-to-grow" designation is less dependent on the vegetable species than it is on the climate and soil requirements of the plant. The sensitive root of the artichoke simply is not going to fare as well in Maine as it is in California, and where asparagus is set into soil will not be the same spot as the garden parsnips. Additionally, appropriately adapting to climate zone will dictate more about overall garden success than any single plant will.
Because of its long growing season, a cauliflower plant has more chance to fail than a vegetable with a shorter maturation. In addition to its relatively cool climate requirements, it also needs consistent care, proper soil, water and drainage and nitrogen-rich fertilizer. It is also susceptible to cabbage worms and, if growth is interrupted by extreme heat or cold, black rot, which will inhibit proper development.
Another vegetable that requires cooler weather, a long growing period (45-to-55 days) and is usually begun indoors is head lettuce. Because it is also susceptible to a variety of pests, it is considered a difficult plant to grow to maturation. Aphids, slugs and other plant bugs thrive on the plants and can destroy them.
While celery isn't necessarily difficult to grow, its gardeners often lose their crop because of a combination of poor soil preparation and impatience. Compost-rich soil must be well drained, fertilized with a pH of 6.0-6.5 and capable of moisture retention. While it requires full sun, like cauliflower and head lettuce it does better in cooler temperatures. Additionally, it needs to fully grow (90-to-120 days) before harvesting. Pulling it too early is usually its failure.
Because artichokes are native to the Mediterranean, it is a common misconception that they are difficult to grow in the US. They certainly are counted among the most difficult vegetables to get started, but gardeners in many climate zones throughout the US have success. Avoiding hot, humid regions would be a good idea. Additionally, the sandy soil they require may not be compatible with other plants. Since their roots are particularly sensitive, they also minght not survive cold winters.
Like artichokes, asparagus plants are perennials. While this can be a plus because established plants can offer years of produce, the beginning is difficult. A gardener can wait three years before he sees any real crop. It is a patient gardener who is willing to painstakingly tend to a single vegetable for that long before seeing results.