I did a fair amount of reading about vaccum tubes back in the day, and my family owned a tube-powered commercial Seeberg 45 RPM jukebox, but I never made the mental leap to all-magnetic "circuits."

This jukebox was in our living room (size of a refrigerator):

https://www.prosoundweb.com/put-another-nickel-in/

One disadvantage of magnetic amplifiers that I can think of is that any heavy-current application would require specialized training to service. For example, I work with 1-phase electricity as needed, but I won't even open a 3-phase panel. Can you imagine poking a screwdriver around in a magnetic circuit? :)

Fun fact: after the first vacuum tube amplifier was patented, patent trolls would solder random collections of vacuum tube logic (VTL) and patent those. But when cross-examined in court while defending their patents, they could not explain how they worked. Kind of like the first spaghetti code, but in hardware.

It would be very cool if somebody made a graphical simulator for magnetic circuits and amplifiers, like Kerbal Space Program (KSP.)