@bernhard YAML is preferable because it's declarative instead of imperative. This has a couple of side-benefits, like cleaner diff views in git, no formatting issues or different styles and no 'noise' in your commits (all only relevant if you have a git-based workflow with pull requests). But the big thing is that it makes it impossible to create environment-specific configuration, which is exactly what you don't want. If you embrace that the configuration is the source of truth for the entire site state (excluding content), you won't need this anyway. Take your example where you switch a field based on whether the languages module is installed - I would flag this in a PR and consider it an antipattern. Whether a site is multi-language or not should be part of the configuration. If it isn't there's no way to guarantee that the code which works in staging will also work in production, so at that point you're doing all the work for controlled deployments and version control but not getting the benefits.
Another downside of PHP is that it's onedirectional by default. With YAML, if a deployment fails, you can just roll back to the earlier version and apply the configuration of that version. With PHP, this may work if the PHP migration is just one single $rm->migrate call with an array of configuration (so basically it is a declarative config). But you have no guarantees that it will, and if you have any logic in your migration that depends on the previous state of the site to migrate to a new state, this migration is irreversible.
Migrations do have their place - if you really need to perform some logic, like moving content from one field or format to another. But besides that, declarative configuration files are preferable.