Welcome to ITKE, devimata! First off, I think the biggest issue is related to data breaches. It doesn’t take a hacker too long to find one flaw in a system. And he/she finds it, there goes all your data.
Secondly, I really believe insider threats can be very harmful when it comes to cloud security. Whether it’s an old employee or contractor, they have insightful knowledge on your system and could very easily cause havoc in your cloud.