Studies comparing patients with the standard length incision and the new shorter incision are now coming available. The results are, indeed, mixed. But there may be some very real ways to explain the differences.
Patient selection is a big factor. Obesity, previous hip surgeries, and smoking are all risk factors that might keep a patient from having a good result. Sometimes the size of the incisions makes a difference. For example, The results won't be much different if the minimal length isn't much different from the standard length.
A recent study from a surgeon who has done over 1300 minimally invasive hip surgeries may shed some light on this topic. In this study, one surgeon, one anesthesiologist, and one physical therapist saw all patients. With the same staff for all patients, data from both groups of patients (short and long incision) was uniform.
The minimally invasive group had less bleeding during the operation. They were also less likely to limp during the first six weeks of recovery. There was no difference between groups by the end of one and two years.
Experienced surgeons say the minimally invasive method is safe. A general consensus still isn't out one way or another.