Although from a US perspective, so some elements need translation, this article on how the quality of firms should be measured provides a valuable challenge to traditional assumptions about size, profitability and history as proxies for quality of advice.

The most important point is the one quoted below. Nothing a firm says about itself, nor any of the metrics (such as PPP) that describe the firm without external referents, should be taken seriously. The only measure of quality is how successfully the work it does meets clients' needs. And clients are the only ones who can define 'need' and 'success' in this context.