Due to Kirby’s ability to infinitely jump, its very easy to just run through most levels. However, if you are looking for secrets you might die sometimes, and some of the bosses are tough in some of the games. The only games i have owned are super star and 64.
In super star this difficulty set up is pretty obvious, as the first and easiest game had almost no secrets to find and only a few easy bosses. The hardest levels were full of both secrets and bosses (the great cave offensive and the night sky) and the hardest challenge was the arena, which is just a bunch of bosses with limited healing.
All I remember from 64 is dying alot. Its been a few years, but I remember the bosses being kinda tough, and the secrets were very rough as they required a very specific ability combination, making 100% very difficult. I don’t think i ever beat it though. The lava boss was too hard