Over on homebuiltairplanes.com there's a retired automotive engineer whose job was analyzing engines and transmissions for torsional vibration, which is the killer of PSRUs. He can quote endless math showing what happens when the thing is not competently designed, and the "successful" PSRUs are more a result of luck than of competent design. Even the RAF redrive I had on the Subaru I installed in a Glastar had its issues, TV among them. There were certain RPMs where you passed through, increasing or decreasing, and did not linger there or bad stuff would happen.
Most people do not understand torsional vibration at all, and that's where the problems start. It is a BIG problem, and it's why most Continentals and Lycomings are direct-drive. They have had geared engines, they had problems with them, sometimes due to pilots ignoring POH instructions on RPM ranges. It is not easy to build one that works safely or for a long time, and so most are overbuilt so as to survive, and that adds weight. Most certified geared engines had splined quill shafts hidden within the crank nose and drive gear to absorb TV. Even the Merlin had that. It's a lot of machining, a lot of calculations, a lot of metallurgy and heat-treatment to get that quill so it will flex forever without snapping. It's a small shaft that takes tremendous torque.
This is an automotive clutch disc:
View attachment 129571
Do you know why those springs are in it? The splined hole in the center is in a plate between the layers of the disc assembly, and it has rectangular slots that those springs fit in. The disc is turned by the engine's flywheel, and transmits the torque via those springs to the splined output plate to the transmission. Other firing-pulse-absorbing parts include the massive, heavy flywheel, the transmission's input shaft, and the wheel driving axles. In a car with an automatic transmission, a fluid clutch called a torque converter does the job of those clutch springs. Transmission fluid between driving and driven turbine discs transmit the torque.
So NO, the airplane and car are different. Thinking simplistically about this stuff is what gets so many builders in trouble. Parts of the RAF redrive hassles were the substitution of a light aluminum flywheel for the original cast iron flywheel, the clutch disc, and the clutch pressure plate. That weight adds up mightily.