The GE UDF was not only unducted, it was ungeared. Two interposed, counterrotating (contrarotating across the pond) turbines directly drove the two fan rotors.
I knew Art Adamson, the genius engineer who invented this configuration. He was personable, played tennis into his 90s, always loved to talk about his introduction to aviation as a pre-teen in farm country.
Only disadvantage I could see was - It had four separate rotors. Per FAR 25, each engine required four tachs. . .
