The basic idea of why people get stuck in the dark night on Goenka retreats has to do with the movement of attention, where insight investigation needs to take place, and the generation of the hindrances from too much tension/contraction.
First, the arc of attention over the insight stages:
Attention gravitates to the centre in Arising and Passing away. This means that where one can perceive clearly is in the centre with the periphery unclear. As one moves into the dark night stages (Dissolution, Fear, Misery, Disgust, Desire for Deliverance, Re-Observation) attention naturally gravitates increasingly outward to the periphery. This entails the centre being unclear while further and further into the periphery gains increasing perceptual clarity. When one finally reaches Equanimity one gets perceptual clarity in BOTH the centre and the periphery. From here the meditator is poised to go for stream entry if this hasn’t been obtained.
So, what does this have to do with Goenka?
After initially getting concentrated with the breath, Goenka then has you doing hardcore single point precision focusing and then precision body scanning. These two techniques both involve a heavy focus on the centre of attention. This is great up until Arising and Passing away, but as was noted above, after that however, perceptual clarity starts fading at the centre and moves outward. So how does this look to a Goenka meditator? Like their capacity to see body sensations is getting worse and worse due to their focus on the centre. They may think thus that they need to ‘up the effort’ or just start to get frustrated. Both of these reactions will increase contraction/tension in the mind which feels crappy and is likely to bring on more of the hindrances impeding good practice. Furthermore, since one has to increasing notice the Three Characteristics in the periphery of awareness rather than at the centre, yet Goenka wants that tight focus on the centre of attention, the meditator is not working where they need to in order to make further insight progress. The unfortunately common result of all this is that the meditator is unable to reach equanimity on Goenka retreats if they closely follow Goenka’s instructions.
Once the dark night is hit, attention will naturally move outward (where sensations naturally will tend to be noticed). For further insight progress the technique has to work with this. Noting (or just noticing if concentrated enough) and fire kasina (if you are concentrated enough to hold the murk as a meditation object) are going to work far better here. Essentially, any practice that moves with attention to the periphery and calls for the meditator to notice the Three Characteristics is going to work to some degree and is a far better choice than the laser-like Goenka approach for getting through the dark night.
Most or all of this explanation was originally touched on somewhere on the Dharma Overground by a long-term Goenka retreatant, so I can’t take credit for working out the explanation. I’ll include the forum link in the future if I am able to find it. For more information on the stages of insight and the movement of attention have a look at the insight section of Daniel Ingram’s book Mastering the Core Teachings of the Buddha (link).