One of the reasons for Fetrons emergence was the potential lower noise and lower distortion vs. tubes. Scarcity of tubes, not so much. (Contrary to what was written in most amateur literature at the time). But of course it depends greatly on the circuit topology and application it's used in. Fetrons came into being for submarine sonar applications where low-noise low-distortion circuits were crucial to improving performance in existing tube electronics of the day. Today they are an interesting esoteric vestigial remnant of the era where hollow state was giving way to solid state. How practical they are or whether they have any useful application over more modern equivalent circuits that can be built today is dubious at best.