How can I automatically assign users to a node based on a taxonomy?
I would do something like this:


RFP question is a custom content type.
RFP question includes a term reference/entity reference to a taxonomy vocabulary called Category.
Category has an entity reference field (to users) called SME.  This way, each term can be assigned to a user.  If you do it this way, you also don't need to worry about updating existing nodes when the SME changes; you can simply update the term entry in the vocabulary Category once, and all relevant nodes will be changed.
With this config, the SME won't be visible on the node, just the category.  You can fix this by creating a block in Views and adding it to the node page.
You can use Flag with Rules to automatically notify by email the user marked as SME when a node is flagged for updating.