Demo is never accurate for manual trading since there is no slippage, limits are often instantly filled when price is touched and maybe most important of all psychology is completely different (fomo, taking profits early, moving stops, ...).
If the system is 100% automated an slippage is included in the calculations then the results can be accurate, for manual trading it is not reliable.
I'd say psychology is the biggest factor demo vs live. But when using the rithmic demo paper trading, there was not a very wide margin between the demo results and live results (trading both at the same time in the same environment).
Nothing even remotely close that would change the macro outcome of demo vs live. Not saying it's that way with every demo system or that someone's strategy couldn't be so tight that any deviation of that would turn it into a losing strategy. Just saying that was my experience.
