What are the biggest disadvantages of cloud computing?

None of these statements apply to the major public cloud providers. However, I have reservations about entrusting my company's data to Ali Cloud.

The challenges with public cloud aren't those listed above. Security, reliability and complexity is all depending on the configuration and architecture of the solutions, and can be equally disastrous - if not more - if done incorrectly on-premise.

Challenges are training and governing the wide set of services that cloud provides. In the past, a strongly governed organization could limit the number of technology standards because introducing a new one takes a significant initial investment. With the cloud, such technologies can be introduced much more quickly, and properly governing the positioning, control and management of these technologies is actually more challenging to accomplish given how "easy" it is to enable them (or perceived by others).

Another challenge is keeping the cost of public cloud under control. Small projects can have significant OPEX costs associated with it, especially if cost control isn't taken into account from the beginning. For on-premise your cost is mostly sort-of capped by the license constraints, infrastructure capacity needs, etc. (not always, and gone are the days of perpetual licenses).

Using public cloud provides a great increase in flexibility, at the cost that comes from that great flexibility. If you don't organize yourself for it, you're going to get massive challenges on all possible areas. But if you do organize, change the developer and operations people's cultural mindset, etc. then it can be a great enabler.

The biggest disadvantage is you need to re-educate people. Without this, you'll have all three disadvantages. Educate your staff and you'll have neither of them. 

I don think these are big disadvantages.  on prem can also be less secure than cloud - its all about how you configure it.  

I don't agree that the Cloud is less secure than most on-premises sites. I don't think most datacenters have the rigor or the scrutiny that the major cloud providers have whether it is technical access control or physical security. Most cloud models have horizontal security frameworks that allow you to use centralized policy for access control. Centralized policy for all services, and resources for on-premises is nearly unheard of.

Cloud providers also have the architecture to support availability and reliability if you so choose to use it. Most companies can't afford the resources or the time to design, construct, and maintain the global centers that the cloud service providers have. And for most companies their mission isn't maintaining the environment but selling their product or service.

You will always have multiple vendors no matter whether it is hardware or software. With most cloud providers their responsibility model hides you from much of that.

Maintenance is another part of what you are paying for. On-premises requires patching, upgrades, security fixes, vulnerability monitoring, configuration maintenance, hardware compatibility, and a host of other tasks that are expensive and more importantly distracting from business delivery.

You also add complexity by having a hybrid model but that is a different discussion.