>> Why are hydrofoils inherently impractical?

Limitations on sea states. This gets dangerous when the waves very suddenly start striking the hull. The hull doesn't impact the bottom of the wave but mid-wave, slamming into it in a way a normal hull never would. Limitations on cargo capacity (too heavy and it won't 'fly') which is why large hydrofoils are normally limited to passenger services. Increased draft at low speeds (ie when not flying). Moving parts below the waterline are always a fouling hazard. When that delicate foil hits something it doesn't end well. Outboard engines are mounted in a way to survive such events by folding backwards. A lifting foil cannot do that.

These all lead to a dangerous senerio. Say you are driving a hydrofoil in increasingly bad conditions. Because you are 'flying' slightly above the waves you don't notice that they are getting worse. Then one slams into the hull at full speed, or something breaks, and you can no longer 'fly'. The non-foil boat would have turned back long ago, but because you were flying above most of the waves you are now deep into a situation your craft was never meant to handle.