Why there is a USI restriction on Join Indexes.

Database

Why there is a USI restriction on Join Indexes.

-- I have read that USI cannot be created on Join Indexes. Any specific reason why USI cannot be created? Please explain me in detail.

-- I also read that changes to USI is Transient Journaled and NUSI is not transient Journaled. Is it changes in data values of USI/NUSI columns or changes in Columns(for ex: col1 was USI and now we have changed to col2). 

Any detailed explanation on both these questions will be highly appreciated.

Best Regards,

Shankar