Well for one there's being able to control 51% of bitcoin[1] which would let China "prevent transactions of their choosing from gaining any confirmations, thus making them invalid, potentially preventing people from sending Bitcoins between addresses. They could also reverse transactions they send during the time they are in control (allowing double spend transactions), and they could potentially prevent other miners from finding any blocks for a short period of time."

[1] https://learncryptography.com/cryptocurrency/51-attack