I'd like to point out the factors that did exist in the successful PRC invasion of the island of the ROC-held Hainan in early 1950 that did not exist in considering a possible PRC invasion of Taiwan during the same period:

• There was a relatively large and organized Communist armed force already in Hainan that did not exist in Taiwan. The Communists in Hainan, which was occupied by the Japanese from 1939-1945, conducted guerrilla operations against the occupying Japanese, who were hated by the population for killing 1/3 of males on the island. In Taiwan, on the other hand, most of the population really didn't mind the Japanese so much - and a non-negligible amount of males there actually served in the Japanese armed forces in the war. So there was not an effective Communist guerrilla force in Taiwan to support an invading force, as there was in Hainan.
• The Hainan operation occurred in the months immediately prior to the Korean War, when there was a much lower possibility of US intervention. In fact, a few months earlier in January 1950, US President Truman had said that the US would stop intervening in the Chinese Civil War, although it still recognized the ROC as China's legitimate government. These months in between January 1950 and the start of the Korean War would be as good as conditions would get for a PRC naval assault.
• The Taiwan straits is wider across and with more difficult conditions for landing than the body of water between Hainan and the Guangzhou mainland, which was in fact narrow enough that land-based artillery from the mainland could help somewhat neutralize the effectiveness of Taiwanese naval units operating there.
• Thanks to the infrastructure work of the Japanese during their long occupation of Taiwan, Taiwan was just much more developed and thus strategically important than Hainan. The ROC would devote all its resources to the defense of Taiwan in a way that they wouldn't to the defense of Hainan.