95% of the time I lose out in prelims of Cup, to a team a level or 3 above me.

If you are facing a lower level team, you should be able to win, especially if you spruce up your team for the 2nd leg.

I also think the Cup matches have different roll of the dice than League or CL, with home and away seeming quite different too. But as I said all my Cup has been same level or above me, I have never faced a lower level team yet.