The way gearshifting is currently done is very annoying for me. I do not flat shift, I lift off - I have no choice, I have 3 pedals and a shifter, so flat shifting feels wrong and unrealistic. But this makes me slower, simply because I am going for a more realistic approach. I don't think this is fair.
The other thing that nags me is you can't stall the car because the silly auto clutch won't let you, but thats another topic.
IRL I did used to flat shift my old Nissan Micra, but that engine was so slow to pick up that it never harmed it or the clutch. Never done it to more than that tin can though.