Tbh, none of my games that I didn't finish would be attributed to not being finished due to "too hard", a lot of people are posting that they didn't finish due to glitches, bored, scratched CD's, couldn't be bothered, or flat-out didn't like the game.
To me, the only game that falls in the category that the OP is talking about is:
- Battletoads on the Genesis.
- Syndicate on PC (anyone remember that? Came out in '93)
Other games that fall into the category of 'bored and put down' are:
-Ninja Gaiden
-Alan Wake
-Dead Rising 1 & 2
-Splatterhouse
Not saying that I'll never go back to them, they are all great games, but just on the backboard next to other great titles out today