Why questionable? Because they are barely usable normally, indeed, they only cover niche cases. And why illegal? Because they can't (allegedly, I guess?) be uploaded to the section due to how huge are them. However, they're not completely unusable, and can fulfill certain niche cases that maybe some people will find to use. Just take a look to see what I mean:
Mario's Time Machine - Title Screen

This one uses FG2, FG3, BG1, BG2 and BG3, but only 11 colors. With only FG1 free to use for something else, barely can be usable in levels, but maybe can be useful for a title screen?

Download
Donkey Kong Country 2 and 3 - Bonus Screen

These one uses all available slots (FG1,FG2, FG3, BG1, BG2 and BG3) and very few palette colors (DKC2's uses 2 full palette rows, but DKC3's only 12 colors). With basically no actual space to use on anything else, they is barely usable under normal circumstances. Unless you do some workarrounds: put some words in a custom layer 3 tilemap, put sprites in screen to simulate the game's original bonus screens and use an uberasm to make it actually work as a cutscene. With all of this, a basic recreation can be made:

And gif so you can see how transitions go:

(you can tell the fact stuff is on different layers is noticeable due to how they appear/disappear)
Download
Download
So, it's up to you to actually find an use to these graphics. I know there are crazy hack makers here, so I expect them to be used at least once!
Mario's Time Machine - Title Screen
This one uses FG2, FG3, BG1, BG2 and BG3, but only 11 colors. With only FG1 free to use for something else, barely can be usable in levels, but maybe can be useful for a title screen?
Download
Donkey Kong Country 2 and 3 - Bonus Screen
These one uses all available slots (FG1,FG2, FG3, BG1, BG2 and BG3) and very few palette colors (DKC2's uses 2 full palette rows, but DKC3's only 12 colors). With basically no actual space to use on anything else, they is barely usable under normal circumstances. Unless you do some workarrounds: put some words in a custom layer 3 tilemap, put sprites in screen to simulate the game's original bonus screens and use an uberasm to make it actually work as a cutscene. With all of this, a basic recreation can be made:
And gif so you can see how transitions go:
(you can tell the fact stuff is on different layers is noticeable due to how they appear/disappear)
Download
Download
So, it's up to you to actually find an use to these graphics. I know there are crazy hack makers here, so I expect them to be used at least once!