The end goal of mindfulness is to make you realize that you don't exist and that you have no free will, and there is no "you." With enough practice you will get there, and it's interesting to be able to meditate to the point where you briefly experience periods where you don't exist. Overall, while of course it's true that we have no free will and there is no "you", it's a terrible place to leave someone, especially…
I wonder if part of the problem is exposing people to a visceral understanding of that without also teaching them an intellectual understanding. I.e. that if the universe is deterministic, then all their actions were predetermined at the start of the universe, and there's no sense in worrying about the future because there's nothing we can do to change it. And to the extent that the universe is non-deterministic, the added randomness means we have even less (still zero) influence over future outcomes. In this light, the notion of "self" drops away as an arbitrary distinction among cogs in a single machine, and without the self there's no way to think thoughts like "I am sad" or "I am unhappy". Because such thoughts rely on belief in the existence of "I".
I can imagine it causing trauma when someone suddenly feels via meditation that they have no "I", but have no way of understanding/processing this on an intellectual level.