What is the actual failure rate of hair transplants?

There is no clear data on hair transplant failure rates. It depends on the doctor and clinic.  Some doctors may have a high failure rate and some may have a low failure rate.  There is no such thing as 100% success rate no matter how good the doctor may be. 

It really all depends by what you mean by "failure".  If you mean that the hair grafts don't survive the transplant procedure (just like not all skin grafts or bone grafts succeed) and die a few days after surgery, then the failure rate for an individual grafted follicular unit  is well below 5%, and probably less than 2%, as long as the surgeon used careful technique to harvest the hair, and as long as individual units are implanted using the FUT technique (which everyone should be  using by now).  The reason for this is that the individual grafts are very, very small, and the scalp has great blood supply.  The smaller the volume of tissue being grafted (in this case just the hair shaft, the hair bulb and the small amount of tissue surrounding them) the higher the rate of graft survival.  The grafts are only vulnerable for the first 5-7 days after the procedure.  After that they become well-attached to the scalp and have a good blood supply.If by failure you mean that the hairs don't grow well after the procedure is done, that can depend on many factors, but probably the most important are that the patient truly has hereditary hair loss (androgenetic alopecia) as opposed to another, less common cause of hair loss, such as a chronic disease, scarring conditions, or autoimmune disease, and that the hair grafts were harvested from a stable part of the scalp that is resistant to future hair loss.  In conditions other than hereditary hair loss, the scalp blood supply may be poor, or the tissue may be too fragile to support grafting - patients with these conditions will probably not do well and should not have a transplant.  If the procedure is done in a very young patient (say under 25) where it is very difficult to predict where the stable hair will be in 5 - 10 years, then it is possible that a significant percentage of the grafts may be harvested from an unstable part of the scalp and not grow well down the road.Another reason some grafts may not grow is if the hair follicle is injured (transected) during the FUE harvest technique.  No one is perfect. A careful surgeon or technician can harvest an entire donor site and expect less than 4% follicle injury rate.  Interestingly, the robotized ARTAS machine has a higher transection rate, probably closer to 20%, so you lose  more grafts. Transected grafts are usually picked out by an experienced technician before implantation and discarded, so they don't really impact the survival of implanted grafts.Fortunately, a good, experienced surgeon will look out for all these things, and will be able to predict quite accurately which patients should not have a hair transplant for those reasons.  If you don't have any of the conditions we talked about, you can expect over 95% graft survival and growth.That's a very long answer to a short question. I hope it helps.

The hair transplants when done in good hands by experienced teams will survive in the 90%+ range.