Two massive problems with using a reel mower
1) frequency of mowing.
When we had one mowing had to be done almost daily if we had a wet growing season
2) type of grass
We grew different types of grasses then. very much couch and similar with fine leaves that cut easily.
The modern wide leaf tough grasses tend to be too thick and soft to cut efficiently with a reel mower, powered or unpowered.
On top of that as mentioned the bed knife needs to be razor sharp.
The reel knives are not so important to be sharp but they must be true to the bed knife, close fitting and have absolutely no knicks or burrs.
This is the same for all reel mowere not just self powered ones but the self powered are far more sensative to sharpness.
They work best with the type of grasses used on golf greens.
The other bugbear is the need to rake all twiggs & fallen leaves off the grass before you mow as twiggs will dent the blades.