I detect only two ways in which virtual reality can be harmful for us.
The first is probably the most obvious. When your eyes completely focus on the virtual screen and do not sense your physical surroundings, you might collide yourself with these surroundings if you do not make enough room for you to navigate. These collisions might also injure you if you are, for example, to fall off while you’re standing, because you didn’t detect a nearby object that made you fall.
The second way it could harm us is yet to have arrived in our times, but when it will it can be quite horrific. If VR technology is to advance so much to the point that the graphics will be realistic, in the same amount of realism as in actual reality, we can have the probability of failing to distinguish between virtual reality and actual reality, potentially trapping us in a constant metaphysical doubt of trying to decide in which reality do we actually experience and belong.
This sense of psychological horror can be compared to the horror some people may feel after receiving a significant amount of facial surgery. When you have lived your life so far knowing how you look like, and then you find yourself to have a completely different face from the face you identified to be yours, can lead to a identity crisis, equivalent to that of a really advanced VR technology.
And indeed, one of the things that lead to the feeling of horror is not knowing who or where exactly you are. This sense of ignorance about how you look like and/or where are you actually in, can often lead to people feel scared, because these details you’re missing are so elementary to our daily functioning, that without them you are doomed to a potential insanity.
Compare this feeling of horror to the horror of Silent Hill 4: The Room. Especially in the beginning, you have no idea where you are and what the hell happened to your apartment. The door out of the apartment is locked with many locks FROM THE INSIDE, somehow there is this huge hole in your bathroom that leads to you various places you have never seen before, and everytime you find a hole that brings you back home, you return to your apartment by waking up in your bed. This illogical absurdity is, in my opinion, one of the most horrific metaphysics I have yet to encounter in any media; a horror that can be equivalent to a potentially-dangerous advanced VR simulations.