There are a few issues to consider.
Dedicated off road vehicles tend to have ladder or in some cases space-frame chassis. There are robust and make it easier to add specialist bodies but tend to be heavier than equivalent monocoque designs. Equally they may also need roll protection which adds further weight.
They tend to have high displacement engines for low-end torque, again these are heavier and tend to have worse fuel economy compared to the trend in road vehicles which is towards smaller higher revving engines.
Four wheel drive transmission is inevitably heavier, bulkier and more complex than two wheel drive, especially front wheel drive. You need extra drive shafts, differentials and prop shafts. Dedicated off road vehicles may also have low range gearboxes and diff locking mechanisms all of which add weight, complexity and transmission losses.
Off road tyres tend to have more rolling resistance than road tyres due to a more aggressive tread pattern and larger and heavier wheels and tyres overall.
They often have large frontal areas, increasing aerodynamic drag and combined with gear ratios optimised for traction this tends to reduce fuel economy at road speeds.
Parts in general will tend to be heavier and more robust to cope better with shock loading and there will often be extra components such as sump guards and other under-body skid plates as well as snorkels, winches etc etc.
While not directly related to efficiency off road vehicles tend to have high centres of mass and soft suspension with a lot of travel which tends to compromise their handling in normal road conditions.
Many of these issues can be overcome at least to some extent by the use of active suspension, lightweight materials etc etc all of course at the expense of increased cost.
Of course 'off road' is a pretty broad category and can range from relatively lightweight utility vehicles to heavy plant.
There is also the consideration that an off-road vehicle has a 'hard' minimum performance requirement in order to fulfil its role, compared to road cars where performance is more of an incremental scale. So there is less design scope to make compromises between efficiency or fuel economy and performance.
From a control perspective key areas are traction control and suspension.
Traction control has the advantage that there is a lot you can do with existing hardware and can include
- decent control
- anti-wheel-spin / torque distribution
Active suspension requires a bit more on terms of hardware design