What are the most common equipment failures that cause boats to drop out of the race?

Rudder failures and fresh water system failures.

A good emergency rudder or other backup steering system will bring the boat safely back to California or to Hawaii, but is generally not sufficient to keep racing.

Fresh water system failures can be disastrous if the installed tanks contain the only supply of drinking water. Even with multiple tanks, failures in cross-connect plumbing have resulted in all tanks being drained. It is advisable to keep sufficient drinking water in containers that are completely isolated from the boat's fresh water system.