Avantages
People are friendly, and passionate about their work. The work itself is interesting and rewarding. There are, indeed, opportunities to advance professionally (at least in my experience). If you make yourself useful in new ways, and develop new skills, you can get new responsibilities and a better salary. Management, at least in my experience, actually cares about staff, and they give you the flexibility to live your life and to advance in your career without losing a healthy balance. The benefits are good, and the connection to the University offers several advantages. Salary is honestly not that bad, though there is certainly a major discrepancy between lower and upper-level staff - most likely not fully deserved, in some cases. But some (many?) parts of WDI are well-run by competent and caring managers - the unrelenting negativity in some of these reviews may well be from several individuals who all had the same (bad) manager - who may well have moved on by now. (A few problematic managers have left the organization in the past few years.)
Inconvénients
Yes, there is a significant discrepancy in salary between staff, and this clearly causes bitterness among many (for good reason). Salary doesn't always seem to correlate to hard work or contributions to the institute. Upper management isn't always able to communicate a clear vision to staff, and often gets bogged down in the academic weeds when talking about the Institute's work. There is also a pretty clear preference for initiatives that align with the president's personal background, interest and focus on healthcare, though that's only one part of what the Institute does.