what is the disadvantage of a container, only hear advantages

There are most certainly a lot of advantages to using containerization technology as part of your platform, and it’s worth exploring when building new solutions. But as you point out, it may not always be the most suitable.

In terms of Security, it not always be suitable to have containers running on the same machine as each other. This is normally not an issue for most organizations, but you can imagine for Managed Service Providers, who may be hosting solutions for multiple customers, there may be specific security requirements that would make having containers on the same underlying host as being potentially unacceptable.

Certain technology stacks may also not be able to run in containers, or would be extremely difficult to refit into containers. This might include very complex legacy applications, which would not benefit from running in a container.