It's a nightmare when it comes to the relevance of all those things, with an extremely long tail of queries, ranging from the lunch menu to narrower, domain specific knowledgeable questions about the company's domain. There's usually not enough traffic to figure out what a good result is for a given query. And users would usually rather complain than be part of the solution. Throw on top of that doing relevance on top of federated results is a big headache...
Very different than a consumer-facing e-commerce product search, or searching your blog, etc
Very different than a consumer-facing e-commerce product search, or searching your blog, etc