I grew up in the era of boss rush segments towards the end of a game to add filler and absolutely hated them.

FWIW, I'm also much bigger fan of difficult levels over difficult bosses.

I also think that the level of perceived difficulty depending on the player plays a large part in the enjoyment. When I play a video game, I'm also imagining that I'm actually that character so every death essentially means that I was defeated in that world for real.

I absolutely hate games where one of the win conditions is to be forced to die numerous times to discover an otherwise inscrutable pattern.