Just hovering is less inefficient than you might think if the surface is smooth. The big use of power and fuel is shoving yourself along with a propeller. The sidewall craft used structural sidewalls sticking down into the water which had water jets in them which made providing thrust a bit easier but of course increased drag immensely compared with a pure hovercraft, even though they only dipped their sidewalls in to water a bit (surface effect ships use the same principal to reduce drag immensely by pressurizing the volume between catamaran hulls to buoy the vehicle up a bit and reduce displacement and drag).
As might be expected hovering on an inclined surface means generating thrust just to stand still. A solid surface also rules out sidewall craft, of course. In the 1960s the Saunders Roe company in the UK got nationalized and became the British Hovercraft Corporation (probably not quite in that order). Their R&D department developed ideas for land-based hovercraft that used various methods to provide thrust. By compromising the efficiency somewhat some weight can be taken by a wheel or track and some thrust provided by it. By increasing lift temporarily this design could hop over quite large obstacles at speed. (They never built a full scale example to my knowledge.)
Pure hovercraft are quite inefficient due to blowing themselves along, hence the demise of the turbine-powered vehicles and the development of the diesel powered vehicles, which became feasible when low-pressure skirt systems were developed (I mean big vehicles here, as little sport vehicles have long used IC engines). However, the inefficiency is not due to hovering. An aircraft needs full power to drag itself up to takeoff speed and pure hovercraft at speed are a bit like planes constantly taking off. At lower speeds and on gentle slopes they are like an aircraft taxiing; not as inefficient as you might think but not much of a competitor for vehicles with other means of providing thrust. (Not sure what other vehicles blow themselves along; there was that land yacht in Barbarella?)
To answer BlackMelon's question slightly more directly a pure hovercraft is not suited to slopes (to hover up the sloping ramps that hovercraft typically use over beaches before landing and disgorging passengers they usually use moderate velocity over water before the ramp and use the slope to slow down (carefully!)). They are more efficient than another vehicle such as a plane blowing itself up a hill on wheels but less efficient than most other vehicles. One of Saunders Roe's hybrid ideas would have been in between. A pure hovercraft is passably efficient compared with a displacement vessel if high speed over/through a water is needed. (If for some reason a downward sloping surface was travelled over then they would be quite efficient for quite large payloads.) Ornerynorsk's hoverpads were an early development and as he says they work very well on the flat, but heaven help you if you've got something heavy on a slope. A niche use that I'm not aware of hovercraft being used for is places like the playa in the Black Rock desert of world landspeed fame. This becomes absolutely impassable when wet in the winter and a little hovercraft would maintain easy communication across the valley.
