These are only underperfoming if winning the GF is all that matters (yes i'm aware that is the ulitmate goal).Cats from '89 - '97. 4 Grand finals in 7 years without a premiership. A number of other heartbreaking finishes too. This period still haunts me. Don't think I'll ever quite get over it no matter happens in the future.
Collingwood 1977 - 1981. 4 Grand Finals (5 if you count the replay in 1977 separately) in 5 years without a Premiership. A number of those were very close loses too from memory.
But, being in multiple GF/Prelims in a span is what you aim for, then you need luck/fortune to win one. It's not easy to win a flag. Are Sains all of a sudden a success if the ball bounces right agains the Pies, or they play 5 monutes more and don't go to a replay?
Cats ran into the tail of an all time Hawks team in 89 and only lost by 6, 92, 94, 95 were West COast and Carlton who were both absolutely stacked.
Underpeforming are teams that should have made prelims/GF's and didn't, or burned out regularly in straight sets.