Are HV/HP switchable pumps as capable as a specific HP or HV pump?
Answer: No.  Any tool intended for a specific job will always be better at that specific job than a multi-purpose tool.

That said, pricing may mean that a single pump is sufficient for your needs.

Fat bike tyres have a lot of volume to fill, but generally they don't go to high pressures - I'd be surprised if you're running them at anything above 40 psi.

Note I assume you're not talking about shock pumps when you say high pressure - they are used on higher end shock dampers and not tyres/tires.

You said "less work" but from a wording point of view, the work remains the same, your input multiplied by the efficiency of the pump will approach the total work required but never equal it.  So a 50% efficient pump will feel like you're doing more work than a 75% efficient pump.