Off-site shredding services put your documents (and your business and customers information) at much more risk and liability than on-site shredding. You are allowing your documents to leave your location and travel out of your sight which then multiple parties will deal with transferring your information to its final unknown shredding location. Also before your documents reach their destination the shredding company will make multiple stops over the day exposing your documents making the situation more compromised with every stop.
Also what if the vehicle breaks down taking your documents to the shredder? That situation offers another element of uncertainty. What happens to your documents if the company has to switch them to another truck or get the truck towed? This scenario must be thought over if you are allowing an off-site shredding company to take your documents.
Once your documents reach their destination then you have to add in the human factor. Who will be sorting through your documents? By this time your documents have been handled multiple times by unknown people You are allowing your most vital information to be taken away, travel hours away to a warehouse, handled by strangers before it is shredded.
Every factor of that equals risk for yourself, your business and your customers.