There is usually a marked difference between "insight" practice (that give realisation about the fundamental nature of experiences) and "concentration" practice (i.e. jhanas). Just practicing jhanas is not all that conducive to mental upset - you purify your behaviour, focus the mind, suppress the hindrances, and experience intense states of wholesome qualities that take over your consciousness for a while. There's nothing negative or dark about that. But Buddhist practice goes beyond jhana practice by using the power of that focused mind and suppressed hindrances to analyse experience through the lens of the Buddhist teachings i.e. right view. The issue comes about if the analysis comes from an incomplete or wrong view, leading to nihilism. It's best to stick to "pure" jhana practice and not meddle with insight practice if possible, if one doesn't have access to reliable teacher than can correct misconceptions in view that lead to negative outcomes.

The other thing is that "pure" jhana practice, when done correctly, kind of sets up the mind with a kind of wholesome "blank slate" from which to do the analysis that leads to insight. Some traditions, especially those that have been imported as "secular mindfulness" to the west, tend to emphasise just doing that analysis of experience without too much jhana practice involved. If you don't already have a very wholesome mental state (i.e. the kind of mental state that comes about from living in a monastery and limiting the actions and speech being undertaken) then there can be a whole lot of mess to sort through and opportunities for unwholesome states of mind to latch on and become the basis for your worldview. It's not a coincidence that a lot of the narrative and commentary about mindfulness and the "dark night of the soul" (i.e. dukkha nanas) comes from those traditions, but isn't mentioned in other traditions that emphasise practice differently.