One complaint is that as the spec stands it's impossible to interoperably implement in browsers just by implementing the spec. You also have to go and do some

One complaint is that as the spec stands it's impossible to interoperably implement in browsers just by implementing the spec. You also have to go and do some out-of-band agreements which may or may not happen. In other words, it's a "standard" that deliberately sets up a situation where behavior across browsers will differ. Suggestions that the interface between the CDM and the browser actually be standardized have been made ... and ignored.