In real use, hierarchy or tag ensures that you'll miss stuff, and what you didn't mistag or forgot to tag or didn't bother to, will still be difficult to find among the noise of a huge taxonomy or hierarchy...
Like what, domain-specific information? If so, that's a question of fine tuning the (machine learning) model. Thereafter, it can automatically tag as it indexes.
Let's say I look up the company benefits and search returns a document from 2015. Is that still relevant, or has it been replaced by a newer policy that search isn't showing? How would I know?
Only for information that has no real value.
Search misses things. Hierarchy or tag ensures that all you have marked can be found.