Views integration - Missing handler for custom fields on relationships broken since upgrade to 4.7.15
Just ran into the issue when upgrading from 4.7.13 to 4.7.16. Spent way too much time trying to figure it out and far too many views data cache clears/drush cc all. 

In case anyone runs across this having the same problem, this is a direct link to the revert patch mentioned above: CRM-19545: Revert CRM-18776 to get back missing custom fields