It is not mindfulness itself that is pseudoscience, but the simplified claims of new/modernized techniques that are put under that name, and their rush to link themselves to state-of the-art neuroscience and experimental psychology.

It is worth repeating the (Wikipedia) definition of mindfulness: "Mindfulness is the practice of purposely bringing one's attention to the present-moment experience without evaluation"

In this raw simple form, mindfulness is always a net positive (in the long term). However, I agree with you that special techniques (and obsession) can lead to negative effects especially when done without guidance. The reason is simple: Any powerful technique/technology is quite harmful when misused, and the mind is a fragile system.

Is there any further reading I can do on this? Or could you elaborate?

Obviously I meditate, but I do see how it could lead to drastic changes in how one exists in the world.

Is the nature of mind best not investigated first hand? The downsides may be mental health issues, but are there any upsides to this kind of practice?