Which of the following is the main disadvantage of extemporaneous speaking?

Extemporaneous speaking is a type of speech contest wherein a contestant is given a topic and asked to prepare a speech after a specific set of minutes, perhaps after 20 minutes of thinking. They used the alloted time to organize their thoughts. Some of the disadvantage of extemporaneous speaking is that the contestant maybe less conscious about time limit and focus more on the speech. The probability of saying things or communicating ideas they don't really mean is high. Moreover, the main disadvantage of which is that the speaker may be prone to use less precise language and sound not-so-fluent-in speaking English due to simultaneously re-organizing and speaking the ideas.