It is difficult in practice to ensure that the target is perpendicular to the axis of shooting (it is however essential).
The tutorial presents the visualization of sufficient and rapid clarity on the rear screen is very optimistic. In reality, it takes a lot of sight near the objetcif (much more than the usual distance shooting) to fine tune (even with bright objectives that reduce the range of sharpness).
It works but it's not as magical as we imagine spending 50 euros.