Here is a JAMA Viewpoint piece expressing a negative opinion about ACOs. (A companion article in the same issue was favorable). Key points:
The authors point out that any cost savings attributable to the ACOs appear to be nominal. Moreover they tend to be offset by the bonuses paid out to the participants. These bonuses cannot be considered optional because they are inherent to the core notion of the ACO: that “quality” will be rewarded.
The model creates incentives to integrate services. Thus smaller hospitals close or are merged, resulting in monopolies among health care delivery systems. This reduces competition and tends to drive costs up, not down.