Thanks for the feedback! Reporting and Analytics team is developing a new tool for building custom report which will enhance the capability and customizability for custom reports. This is in our road map for cloud based solution in Q3 2018 and will come to the on-prem version after that. Please feel free to let us know your specific use cases and features you would like to see in the new version.
We store all reporting data in a SQL database and then report on it from there. I have found the out of box reporting tools in Forms to be a little tedious. Not so much for myself, but for my users.