Unreliability
Unfortunately for NSU, the car developed an early reputation for unreliability, from which it would never escape. The Ro80 engine in particular suffered from construction faults, among many other problems, and some early cars required a rebuilt engine before 30,000 miles (50,000 km), with problems visible as early as 15,000 miles (24,000 kilometres). The three-piece rotor tip seals were made out of the same material. The design fault caused the center section to have higher abrasion at cold starts than the corner pieces and the tip seals could push together, allowing the gas to blow past. With a changed tip seal design this was temporarily solved and with a tip seal center piece made of Ferrotic, the problem was entirely resolved. The fact that the rotary engine design was inherently thirsty (typically 15-18 mpg) and a poor understanding of the Wankel engine by dealers and mechanics did not help this situation. By the 1970 model year, most of the reliabity issues had been resolved, but a necessarily generous warranty policy and damage to the car's reputation had undermined NSU's financial situation irreparably. NSU was acquired by Volkswagen in 1969, and merged with Auto Union to create the modern day Audi company as it is known today.
Second hand Ro80s were virtually worthless in the 1970s due to the well-publicised engine problems, and a common "cure" for an ailing rotary engine was simply to swap it for a Ford V4 "Essex" engine (as found in Mk1 Transits) since it was one of the few engines compact enough to fit in the Ro80's engine bay. Thus in an ironic twist, one of the smoothest engines in the world was replaced by one of the roughest.
The NSU's unpopularity caused by the above problems means that surviving examples are very rare, and are now considered highly-prized classic cars with values to match, particularly thanks to Mazda's perseverance with rotary design, the tip seal problem has been all but eradicated.
Read more about this topic: NSU Ro 80