I've asked the AHJs about NMWU in conduit (besides a short sleeve for mechanical protection) and been shot down. The argument is the cable can't properly dissipate the heat from carrying the current its rated for.
As a result, I've stuck with RW90 in pipe, direct buried and/or protected NMWU, or direct buried ACWU.
I was reading through my Ontario Bulletins from the ESA and found something that relates to using NMWU in underground PVC conduit.
Specifically, bulletin 68-7-4 under Wiring Methods talks about what conductors you can use in a PVC underground conduit.
3.1.2. "Rule 22-200 rerquires individual conductors and non-metallic sheathed cables in Category 1 areas to be suitable for use in wet locations. Non-metallic sheathed cables shall be of the NMW or NMWU type."
So I don't know why it would be allowed in Ontario but not in BC. When you think about it, what you were told doesn't make sense.