I guess the question is "perfect for what?" My understanding is that Lojban is constructed logically so it 'makes sense' to the extent that various signifiers have a rational basis to them rather than the organic, arbitrary way in which a language like English has been put together over the last 1500 years.
It would be good to base your secret dialect or thieves cant or whatever, on this for use as a common language to communicate between people of a similar profession or organization but whose natural language might be markedly different.
The alternative to a constructed language is a pidgin based on the dominant language, but it won't be very secret.
The third alternative is to use an anchient language. Latin basically filled that role among academics from about the 16th to 19th centuries.
A couple other things to consider is writing and hand gestures. Chinese was primarily a written language but everyone speaks it differently. Chinese movies and TV shows are subtitled. . .in Chinese! You can also look at something like ASL to allow for secret communication that no one can overhear and might not even notice if they are not watching closely.
I thought about the idea of a perfect, unambigous language to be used as part of a magic system but that presented a number of problems. One is that if words themselves have magical power, then it stands to reason that anyone could use magic if they can speak the words or draw the runes. That reduces magic to being an exercise in pronunciation and calligraphy that has little to do with the personal characteristics that one normally associates with a magic user: intelligence, wisdom, force of will, essence of soul etc. As the saying goes, you simply say the magic words and there you are.
As a final note, you can't have a perfectly, semantically unabiguous language for use in the real world because the real world itself is not perfect or in any workable way completely digital to the extent every object in the universe can be associated with one and only one word or combination of words so as to distinguish it from every other object in the universe.
Whether you think you can, or you think you can't, you are probably right.