it is piss easy for me. and I played platformers that gave me a lot of trouble in the past.
that's the only measurement I can use here.
Super Meat Boy was hard for me in later levels, and I never was able to finish the dark world completely.
Super Mario 3D World gave me some issues on the final level, but was overall easy.
Super Mario Odyssey's final Darker Side of the Moon level gave me issues. it wasn't that hard tho.
Super Mario Wonder's final secret level gave me more issues than the other Mario levels mentioned, and far more than Astro Bot's final secret level. I'd say 30-40 lifes lost on that level at least... AND I HAD TO DO IT TWICE BECAUSE I DIDN'T GET THE GOLDEN FLAG POLE IN MY FIRST TRY
... second try went smoother tho. that invisibility part is a pain in the ass tho.
I only finished 1 of PoP: The Lost Crown's platforming challenge missions, as they were just too much for me. I almost finished a second one, but after 15 tries or so, I ignored the challenges basically. I didn't have much issue in the main campaign, but there were platforming sequences that I had to try 6-7 times. due to the fast reset points it never was frustrating tho.
Penny's Big Breakaway has some levels that gave me issues. I had trouble with the perspective in that game in some parts, which made judging some jumps really hard, especially with the crazy movement options you got.
some of the bonus levels had me do 4-5 tries to beat, and I died at 2 of the bosses, once on each.
so, that's my experience and how I judge the difficulties of games in relation to eachother. again, that's the only way I can judge Astro Bot's final level... and I found it pretty easy all things considered, while every other level (except that one X level I mentioned) was "piss easy" compared to most platformers I played, most of which were pretty easy themselves.
the 2 factors, 1: you can't fall off platforms, and 2: you can hover and easily control your air movement, are the 2 biggest factors as to why it is this easy I think.