The risks associated with green tea are the same as those associated with any other kind of tea (common black tea, White Tea etc.) and are primarily due to the content of caffeine and tannin. The contraindications may vary with the percentage of caffeine and tannins in it. Most people might be familiar with these adversities, but it is still important to review and summarize the components responsible, as well as the associated risks. 

Tea seeds were first brought to Japan in the early 9th century by the Buddhist monks Saicho and Kukai. During the Heian period (794–1185), Emperor Saga introduced the practice of drinking tea to the imperial family. The Zen Buddhist priest Eisai (1141–1215), founder of the Rinzai school of Buddhism, brought tea seeds from China to plant in various places in Japan. Eisai advocated that all people, not just Buddhist monks and the elite, drink tea for its health benefits.[50]
Rooibos (pronounced ROY-boss) appears to be matching -- and possibly besting -- the health benefits claimed for other more established teas. A favorite among South Africans for years, the beverage is said by some to have 50% more antioxidants than are found in green tea. Antioxidants are the organic substances believed to scavenge "free radicals," the toxic by-product of natural biological processes that can damage cells and lead to cancer.
Shade grown: All matcha is made from shade-grown tea leaves—a labor-intensive process where tea bushes are protected from the sun and light is filtered to the bushes in a very controlled manor. Shading boosts the chlorophyll production in the plant, giving the leaves a rich green color. The lack of sun reduces the plant’s photosynthesis of the leaves, which in turn alters the naturally occurring levels of caffeine, flavanols, sugars, antioxidants, and theanine. By controlling the sun exposure, tea producers can significantly alter the chemical make-up and flavor of the final tea leaves.