What are the disadvantages of having Cerec on the back teeth? Will they be as reliable as other crown options?

There is no disadvantage of having Cerec crowns versus traditional Porcelain fused to metal crowns on back teeth.  If the crown has a good fit and is maintained with good hygiene it should last a long time.

There really are no disadvantages to a crown produced using CEREC.  The process is more efficient and the precision is at least equal if not better than traditional methods.  Patients respond better to digital impressions than having a mouthful of GOOP.  Having the crown delivered in one visit saves both the dentist chair time and the patient time away from work or family, expense and delays to obtain anesthesia.

There are many modern dental labs which will actually use the CEREC system to make a crown using a stone model made from the dental impression.  Some labs are even able to have the digital impression from the CEREC acquisition unit in the doctor's office transmitted to a fabrication unit at the lab. 

The CEREC allows dentists to use modern ceramics, i.e. eMax, which has been shown to be stronger and more durable than traditional porcelain fused to metal crowns.  The CAD/CAM designs produce a far more precise product than the hand crafted crowns made by the lab technician.

I believe that most dental offices will be using a CEREC type system for crowns in the future as this technology becomes a standard of care.

CEREC crowns can have several advantages.  1. The process can be done in one visit, which means less anesthetic.  2. Newer materials like EMax. are extremely durable and long lasting.  3. The aesthetics can rival lab fabricated results.  So the answer to your question is, there are really no draw backs to a CEREC crown on the back teeth.