Disclaimer: This is not meant to be a value comparison between ASL and Cued Speech. I’m sharing my personal experience with both in different areas, and it depends on several factors.
As a complete language, from fluent users, I believe both ASL and Cued Speech are equally viable.
The most important thing for literacy is establishing a complete L1 language, in any language. ASL, English, French, Chinese, Spanish, whatever, it doesn’t matter; just get that L1 language down. The best-performing ASL signers whom I met, at least in academia, usually came from a Deaf signing family, or had great access to complete language models in their educational system.
The big drawback with ASL (or any other sign system) is that most hearing parents have little to no knowledge of sign language, and most never become fluent in ASL. We won’t even get into the morass of all the variants of signed English (or pidgin signed English). For parents, I think Cued Speech’s greatest strength lies in the fact that it enables hearing parents to use their native spoken language, no matter what that language is, so they can act as a complete language model right away.
Another of Cued Speech’s benefits is that it has a much shorter learning curve compared to ASL. Generally speaking, CS takes weeks or months to attain fluency, compared to years for any sign language system. Plus, once you know the system and develop a good knack for thinking of English phonemically, you can cue pretty much anything without needing (much) instruction. I think the mental shift to phonemic English, as opposed to written English, is the hardest for most people to get past. It often trips me up too, and I’m a native cuer!
Earlier this year, I started working with a Vietnamese mother on developing Cued Vietnamese for her deaf daughter. English is her third language, and she wants her daughter to have access to the language of her heritage. Chances are her little girl won’t get that through sign language exposure within the US; how many deaf Vietnamese in the US are there who also know Vietnamese sign language– if there’s even a standardized Vietnamese Sign Language?
Cued Vietnamese, on the other hand, would give her access to spoken Vietnamese– and by extension, all the cultural nuances that language carries.