To be clear, I'm not a scientist, so I was trying to avoid getting into quantum physics, however if I'm not mistaken, there are some processes on the quantum level that seem to be irreversible or wholly unpredictable. As such, going back in time wouldn't guarantee the same outcome on a quantum level. However, afaik the effect that it would have on humans is purely speculative.

Additionally, it could be that our understanding of quantum phenomenon is similar to our understanding of the universe back when astrology was all the rage and people believed the stars and planets were unpredictable. It could be that we're working from the wrong frame of reference and as a result, it only appears as though things in the quantum realm are hard to predict if not straight-up unpredictable.