Most modern airplanes have a backup for every possible problem that may occur, but when the landing gear of an aircraft fails to deploy, the situation is complex.
A lot of the damage caused to a plane in the event of landing gear failure is due to the friction, with the hard ground on the airfield. In such a scenario, it may sound like a good idea to land on the soft grassy ground instead of the concrete airfield but according to an aircraft mechanic, landing on a smooth hard surface is much less damaging.
The grass may be soft, but the surface will neither be even or smooth. Under high pressure, it can cause unpredictable movements causing problems like structure formation due to unequal pressure and bouncing. This can potentially cause fuel leakage and even prevent the doors from opening.
As the airplane body is mostly made of aluminum which is pretty soft, so there is little chance of sparking. The landing gear is made up of harder materials that could make sparks, but since that is not deployed, it is not a problem. The fuel tanks or cells won’t catch fire either if the landing is controlled enough under such a situation.
If the plane manages to land belly first on the tarmac, it is possible to fix the landing gear by removing any problem that prevented it from deploying. If it were to land the same way on grass, it would be significantly harder to fix it with grass all over its parts.
The Boeing 767 made a belly landing on a runway in Warsaw, Polland in 2011, watch the video below.