Please re-read this, which states that:
For example, to deploy a cluster consisting of three peers, one master, and one search head, you need five Splunk Enterprise instances running on five machines connected by a high-speed network. All instances must be at the same Splunk Enterprise version level (for example, 5.0.3). And all machines must be running the same operating system.
So yes, the cluster master is included in this requirement. Why would you chose a different OS?
The likely drawback is that you are running an unsupported configuration if chose to deploy differently.
Hope this helps!
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