Can raynaud's phenomenon ever be truly dangerous, to the point of losing fingers or toes?

Yes. In severe cases of raynaud's gangrene may develop in the finger tips and toes. This is often related to concomitant nicotine abuse and is seen in patients with severe scleroderma. It should be emphasized this is not a common occurrence.
Possibly. Some raynaud's primary may have digital ischemia and followup amputations.