Experience shows they have to be voted up or down without allowing each of the 535 Congresscritters a change to amend it, i.e. force a renegotiation with the other pact members, which won't happen since they've already taken political hits once it became public.
If neither of these process details are acceptable to you, then multi-country trade pacts are not acceptable to you, and a lot of potential compromises where e.g. country A makes a concession that helps country B, which makes one which helps country C, which makes one which helps country A, when no (new) bilateral concessions are possible won't happen.
This of course says nothing about the content of any particular agreement, but that's a separate issue from process.
For example if you limited each treaty to 10 provisions of 10 pages each then that would make the document understandable, AND it would incentivise the backers of important provisions to shut out the garbage if the garbage will sink the whole deal.
As it is we get at least hundreds of pages of garbage hiding among thousands of pages of soporific minutia.
