If you were receiving authentication / protocol errors back from Oracle in the logs, then it would be due to the drivers, they need updating and building into the new version (I tried various methods including the known tricks of changing the sqlnet options but none worked).
I guess the traditional methods (i.e. non-container database format) have some backwards compatibility and tolerance built into them still, but container/pluggable databases are where its at for the future, and I'll be trying to get that working as soon as I can (with the releases Namita refers to) to put it through its paces.