Eddy Currents start to lose efficiency and torque resistance at low RPM's as well, and they generally have fairly high inertia due to the large cast iron flywheel that the magnets have to work against
All of the OEM's run AC motoring dynos these days. AC dynos are top of the line, are extremely low inertia, and they can "motor" or spin the test engine with the ignition off to gather critical friction and pumping loss data. AC dyno's are also very expensive. A typical automotive AC dyno cell will run from $2 to $5 million installed, depending on the HP and RPM requirements, as well as the amount of emission monitoring equipment installed. The latest AC dyno's from Horiba and AVL can even simulate events such as clutch slip or chatter, wheel slip on any surface (ice, gravel, snow, etc) imaginable, or the individual firing torque pulses when in motoring mode. I worked for a small engine OEM for 13 years as a test Engineer, and we had all 3 dyno types. Nothing can touch an AC dyno for its range of capabilities, but most performance shops can't begin to afford one either. Water pumps are cheap, but with the price point comes a lack of capability.
Good luck with 427 build.
