I've been researching the Azure Access Control Service (ACS), and it looks like it's especially good at handling authentication from heterogeneous (configurable) identity providers. Then there are a number of additional scenarios that it appears to support (see for example [ACS How-To's](http://msdn.microsoft.com/en-us/library/windowsazure/gg185939)).

The question I have is the opposite: it would really help me to understand, in order to use it properly, what ACS is **not** good for. What are the limitations of ACS, and/or what are some scenarios where ACS would be inappropriate?

(Assume, for the sake of argument, that I plan to create a - profitable :) - public web API and corresponding web site front-end, hosted in Azure - i.e., that I do care about user identity. If you like, you can further assume that my system will be built using .NET.)

Thanks!