Test Data Management - Subsetting using only percent (%)

My customer is looking to use Data Subsetting to reduce the size of a database for cloning purposes.  They are experiencing issues when trying to create a Subset of an application for all schemas and All Tables which is very large.  For "Rows to include" they choose 10% using the 'Some Rows' radio button.  They are not entering a WHERE clause.

For small databases with a small number of objects, the subsetting works without issue, but they tried to create a subset for large complex environments like Peoplesoft and the process fails.  The customer is working with Oracle Support, but I wanted to find out if there are any recommendations on customers trying to use Subsetting for large complex database with a large number of schemas and Tables.