I think that one of the most common ways by which the devotees of reductive physicalism try to make it appear to be a valid position is by positing a false dichotomy by which they then sneeringly characterize anything that's not simply physical as "mystic."

What makes you think that it is impossible to observe someone else’s consciousness?

The fact that it's an emergent phenomenon with no physical manifestation.

I think we'll be able to (and in fact we already can to some notable degree) track neuronal activity in a brain and map it and interpret it, so we can make reasonably solid guesses regarding its nature - general type, intensity, efficiency and so on - but we can never actually observe its content, since its content is a gestalt formed within and only accessible to the mind that's experiencing it.

There's nothing at all "mystic" about that - it's simple logic and reason.

And, by the bye, it's also much of why actual philosophers rejected reductive physicalism almost a century ago.