So do you think any ERP systems can work (defined as providing a positive return on investment)?
My guess is the success of ERP systems is probably somewhat inversely proportional to the complexity of the system. The less complex the system, the easier it and the existing business processes can be combined, the easier it will be for management to understand and use the tools and metrics and so on, and the lower the general costs are and the more likely that the technical requirements will be met without cutting corners that compromise functionality.
And there's probably a bunch of complex site-specific factors around the skill of management, their ability to comprehend and use metrics, and so on.
I'd guess if you were to graph it with "usefulness" on the Y axis and "complexity" on the X, it would look like some curve that rises quickly with features but plateaus and then drops off as complexity increases.