As for it being easy to learn? When I was a beginner 40 years ago manual was the only option. It was very simple to understand. At any given flash > subject distance only one power level will produce correct exposure. That's pretty easy to grasp no, even without instant replay.
If you try manual flash and use it a few times you come to realize that when you put your lights at the same distances the exposure always is the same. My standard indoor portrait set-up is to use my 580ex flashes in M mode at 1/2 power. I shoot from 8ft with fill on my bracket with the key light placed 5-1/2 ft from the nose of the subject — measured with my arm span. The result it the key light winds up 1 stop brighter creating a 2K+1F:1F 3:1 lighting ratio (reflected). The exposure is always f/8 at ISO 100 because there are no camera controlled variables. I can set up and shoot in minutes without the need for testing or trial and error.
Manual is perhaps less convenient in appearance, but if you understand how it works it is actually more convenient. For example I shot over 300 head shots for a church directory with my pair of 580ex flashes. I tried ETTL ratios but every change in subject clothing and skin tone caused the metering to change the exposure requiring FEC adjustment to get back to correct exposure. By comparison with manual flash once the flash distances were set to the subjects to record the full tonal range I was able to change subjects and not need to twiddle the dials. Every face, white, black and yellow was expose perfectly as were the black suits and white shirt collars. The backgrounds on all the shots matched perfectly. Why? Because I was in control, not the camera.
