I spoke with @jamesob about this. I am no longer affiliated with Blockstream, but I do feel that liquid script can benefit from CTV. I worked on research team, maybe @stevenroose and @instagibbs who worked more closely with liquid can correct me if I am in the wrong here.
You’re right. A straightforward implementation would be highly prone to issues for the same reasons you mentioned. However, this can be addressed as follows: users deposit funds using a standard liquid peg-in script. These peg-in funds are then consolidated into a separate CTV address managed by the watchmen. This shifts the responsibility of correct CTV usage from users to the liquid engineering team.
While this approach requires careful engineering, the potential fee savings could make it well worth the effort.
I don’t think Liana can achieve something similar since it is non-custodial. In this case, once the funds are in the watchmen’s custody, they can optimize spending as needed.
Overall, I agree with the sentiment that general-purpose vaults are better and less error-prone for this use case, for the same reasons you listed. However, as mentioned above, CTV remains useful on its own for avoiding recurring fees.