I think the frustration is how they lost in the playoffs, especially in 2018 and 2019.
2017 Can be excused since no one expected them to make it and every game against Washington was a one goal game, with Games 1, 2, 3, 5, & 6 all going to overtime. So when you look at it like that with a few more bounces of luck when they lost Game 1, 5, & 6 they could have won that series.
However in 2018 they had a 4-3 lead going into the 3rd period of Game 7 and lost 7-4.
In 2019 They had that 3-2 series lead going back to Toronto for Game 6 with a chance to eliminate Boston. They had a 1-0 lead in the 1st period, however the Bruins got back to back power play goals to take a 2-1 lead and won 4-2 forcing Game 7 and we know what happened next.