There was nothing simple about the design of the Wankel engine. The rotor and chamber contour were very precise surfaces which require tighter tolerance control than a piston engine.
It's down fall was two fold. The apex seals required to separate the combustion zones were problematical due to the high surface running speeds of the tips of the rotor and the difficulty of getting any lubricant to them because combustion pressures alternated from one side to the other as the engine turned over. The second problem related to the surface geometry of the combustion space. For it's displacement any Wankel will have a greater surface area in it's combustion chamber then any equivalent piston engine. The larger surface area tends to quench the combustion process. The result of this is lower combustion flame temperatures with a corresponding increase in emissions of unburnt hydrocarbons. That also means that gasoline is going through the engine without burning which means mileage is less than optimal.
Versions of the engine have been used as rotary air compressors with good result, though as an engine it's more of a technical novelty.
Regards,
GtG
Exactly, I believe this problem is probably more fundamental that the seals problem. You could imagine some technical solution to the seals, and many of those engines lived a fairly long life without problems (my father owned a 1979 RX-7, sweet little car..). But the problem you mentioned above is simply unsolvable and to most, its not obvious.
Most people thought (think?) that the fact that it is a ROTARY engine makes it, by definition, more efficient, being that the output you desire (flywheel rotation) is also rotary, as compared to converting the up-and-down motion of pistons into rotation, but that is simply not true.
I suppose time will tell with this engine, I'll believe it when I see it!