When performing an in-place upgrade to the latest version of AEM, there are several potential disadvantages to consider:

Long and Arduous Process:

The complexity and duration of the upgrade process can be significant, especially if there is a considerable version difference between the old and new AEM instances. This is because the upgrade process must account for all the changes and improvements made over multiple versions, including content structures, APIs, and underlying technologies.

References:AEM Upgrade Best Practices

Complex Developer Setup and Automation:

Upgrading AEM involves setting up and configuring the environment, updating custom code, and ensuring that all integrations work seamlessly. Automating these tasks can be complex and time-consuming, requiring significant developer effort to script and validate the upgrade process.

References:Automating AEM Upgrades

These disadvantages highlight the potential challenges and resource investments needed for an in-place upgrade.