I'll start on that one.
“Tax haven” is a term for a jurisdiction that imposes low, minimal or no income tax. It's often corporate income tax, but it can be personal income tax as well. Sometimes it's a special regime that's available only to foreign entities with the objective, occasionally, of attempting to attract investment income from other countries. That's the definition of a tax haven.
You asked, just to get your question clear, about challenges the CRA encounters.