I have given mains with both humanities and engineering optional. I can for certain say that the amount of hardwork required for engineering is too much, compared to that humanities takes lesser time. Also the 3 months gap one has to spend time to get their calculation strategies in order to compensate for the time alloted for prelims this takes a toll on gs.
But it is also true that science optional have clear direction in which the work has to be put in, humanities one is not sure of what to study
