I've been including some private repos in some work I've been doing recently and Yarn has been spitting its dummy out at packages that begin with 'git+ssh'.

The fix locally was to create an ~/.ssh/config file using vim ~/.ssh/config. If the file is not there it will create one. Then add host information and the user:

Host bitbucket.org  
    User git
Host github.com  
    User git

All works fine.

References