The adding of the repository itself is not dangerous.
But you will probably do an
apt update and
apt upgrade (or similar) sometimes after you've added the repository. The
apt upgrade will cause any software already existing on the system to be updated with a newer version if possible - no matter if this newer version comes from the same repository as the previous version.
This means if the new repository you've just added claims to have newer versions of some software you already have on your system, then the newer version from the recently added repository will be used to replace the existing software version.
This way backdoors or just buggy software with exploitable vulnerabilities could be added to your system - and this is definitely dangerous.
Note that the contents of a repository can change after you've added it as trusted. This means even if the repository does not currently contain software which you have on your system already it might contain such software in the future. Thus you should not base your trust on the current contents of the repository but decide if you trust everybody which can make changes to the repository.