Agreeing here with Dr. Farr.
Traditionally, only the Moon is subject to being in the Via Combusta. Here is why (well, at least a brief suggestion of why):
15 degrees of Libra to the end of Libra is the exaltation of Saturn and from the beginning of Scorpio to 15 degrees is the fall of the Moon and a strong placement for Mars. Also, the Sun is in it's fall in Libra and the Moon in its fall in Scorpio. So, in short, the luminaries do not do well in this area of the zodiac, as far as dignity goes.
Now, this does not mean that if you are a LIbra or a Scorpio that you are bad, or that any other planet is un-naturally afflicted. Every planet has it's home like every dog has it's day.
Now, when the Moon is in the Via Combusta, it afflicts the querent with [allegedly] unreasonalble or irrational thoughts. The Moon rules the mind, traditionally, as well as the spirit. So, when the Moon is not well placed the person is "stirred up", so to speak. This is why they warm, traditionally, not to pronounce a judgment when the Moon is in the Via Combusta because it is, well....drama, an unfounded question or something is wrong with the question or the context it is presented in.
It does not mean you absolutely cannot judge the chart...just do not pronounce a judgment or finality to the matter. Give a wide berth to the question, a grain of salt for the querent and follow-up on the matter as it is not so easily resolved. In old movies this is where the show would end with a cliff-hanger and the words, "to be continued....".