Mark,
I’ll share my own experience. The teams I have worked with use the Interlinearizer to produce their back translations. It does produce a very literal translation, which carries with it some of the flavor of the grammar of the local language. When they approve the glosses, they’re exported to the back translation itself. They can always go in and tweak that back translation if it isn’t understandable. This whole process works great for consultants who are able to understand a very literal translation, or who already have some familiarity with the grammar/structure of the languages they’re working with.
If the consultants who will use the back translation will struggle to understand that more literal style, then a manual back translation could be better. If the mother tongue language and the back translation language are too dissimilar, then the Interlinearizer may never produce something very readable, and a manual back translation might be better.
Interlinearizer: Generally quicker, easier to update at each stage of translation. May better reflect the structure or grammar of the original text. May not be clear or natural enough to resolve a consultant’s concerns.
Manual Back Translation: Produces a more natural sounding result, may better convey how the text will be understood. May conceal structural issues or may hide mistakes (the translator can write what they think it should say rather than what they’ve actually written).
I think the main restraints then, will be on who will use the back translation and how clear it needs to be. If the consultants using it will can understand the somewhat literal style of an Interlinearized back translation, and if the back translation does not need to be very clear/natural, then the Interlinearizer is a great choice. If the consultants require a more clear/natural translation, or if the back translation will have other uses that require it to be high quality, then a manual back translation could be better.
One other constraint. The Interlinearizer will work especially well for languages with words with minimal morphemes. For example, with aglutinating languages, where words have many morphemes, the Interlinear requires a bit more work. But for languages where words have few morphemes, Paratext can get very good at guessing correct glosses with minimal extra input. The Bantu languages I work with have many variants due to tense/aspect, subject, object, etc. All of that variation means that Paratext takes longer to guess good glosses for every possible combination. But even so, the Interlinearizer is very helpful in reducing the time we spend working on back translation.