Are there any risks on using 'single-transaction' and 'extended-insert' on mysqldump?
this way is faster because 


Server does not need to perform fsync() multiple times during data import. fsync is the operation of submitting data into disk
Extended insert enables sequential write to disk which means disk does not need to seek for a locations for every row.
If the table has Auto_Increment values, Server can spare adequate values for the batch insertion at once rather than once for every insertion.
The extended insert builds batch INSERT statements (many rows per INSERT). That inherently speeds things up 10-fold because of amortizing the statement overhead (Added By @Rick James in comments)


I know two side effects for this method.


undo log will get too large while performing this operation.
If an error happen during the operation, the entire import will be reverted (which might be an advantage in your case).