It can all depend on the internal IT policies a client has and what skills they possess to which OS may be selected, some clients prefer to be windows based and some try to be as much on unix as possible, it also can come down to cost which drives which OS they select.
As John said it depends on your policies, the existing hardware and the budget that you have available.
In the following url http://www.oracle.com/technetwork/middleware/ias/downloads/fusion-certification-100350.html you will find Oracle's certification matrix.
I would just say that my understanding is that most of the "Non-Windows" OS will not support: DRM, FDM, HFM, Performance Management Architect Dimension Server and File Generator, Production Reporting Teradata Engine, and Strategic Finance. Additionally, Financial Reporting, Web Analysis, FDMEE, Account Reconciliation, Financial Close Management, Disclosure Management, and SOA do not support these platforms when used with Financial Management.
As a result, if you need to create an environment that will include/ support HFM, you will end up with a "windows" environment. Please correct me, if I am wrong,