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

not really. imagine trying to index twitter (internal "social networking"), arxiv (all memos/documents persisted by the company to include insanely asinine stuff like department head site visit schedules, retreat agendas, etc.), wikipedia (dept-specific information silos), and census.gov (budget/other numerical type data), indeed.com (internal opportunities/job postings/transfers)

you also don't have billions of trends/etc. to lean on, nor people's gmail inboxes/web browsing history to scan, so you don't know kathy at accounting only cares about budget memo, and karl at HR only wants department leadership internal job postings, and greg just wants to move out from under his boss ASAP.