Hiring a limo service is safe as long as you hire a reputable limo service that is licensed and insured.
The limo drivers pass background checks before they are hired and should have a license to drive the limo.
I've been in limos before and I took my wife out in a limo on our honeymoon and it was great.
We drove to the wedding in our own car and then I surprised my wife after we got married and had the limo arrive so that we could be drove around in the limo for awhile and then the limo drove us to the hotel where we had our wedding.
I would certainly hire a limo service again as it was well worth the cost and we had a great and safe time in the limo.