My comment on scalability and Windows is mostly based upon cost as well as most distributed software solutions that are designed to scale are primarily done not on Windows Server but on Linux of some flavor. I am also biased here as I started my career in Unix, although I spent ~8 years doing primarily Microsoft development, and am dedicated to non-microsoft environments now. In some cases when clients have demanded we deploy on Windows some of the software vendors would tell us to significantly increase the number of nodes when running on Microsoft as they found their failure rates were higher, not to mention less resource efficient. Hence cost goes up more and maintenance increases as well. And no, I wouldn't necessarily say Windows is as capable as Linux, but that may just be some bias sneaking in.

If you use windows first tools(IIS, MSSQL), windows is a high performer.

But when you get into distributed software solutions which is mainly what I am working on, it is hard to get the same scalability and functionality out of Windows and Windows native products. For example, a decent messaging system like RabbitMQ either has numerous Windows specific quirks or just won't scale the same requiring more servers to maintain throughput and reliability. Not that it can't be done though, just it really changes the cost factor and the architecture. I can even use SQL Server Broker which is pretty good but again, now the costs go up pretty significantly both in terms of hardware and licensing as you can't just run it on a bunch of commodity/virtual servers full or Ram.

I made a great living off Uncle Bill and the entire Microsoft ecosystem, and I am not a Windows hater (I really like C#), I just recognize for most solutions of scale it doesn't work out without significant cost and tradeoffs.