Like the others said, it depends a lot on the song, as both of those genres are generally more complex in rhythm and structure than other genres that get more attention on osu.
I think a big reason those genres are perceived as "hard to map" is because there's not a whole lot of people mapping them in the first place, genres with lots of attention will have lots of maps to look at for guidance, whereas with classical and hiphop, theres not very many high quality maps available to look at, as well as the fact that the rhythm and structure of songs in that genre can vary a lot from one to another, it generally requires more creativity on the mappers part.
At the end of the day, for a beginner mapper who's still learning the ropes, it would be rather hard to map a song in either of those genres.
It would probably be best to learn mapping concepts on more common "easier" genres, and once you have confidence in the editor and a sense of how to map a song in general, you could then give more complicated genres a go.
Obviously none of this is like an "official rule" or anything, if you wanna make a map then make it! But if you're looking to really get into mapping you'd have an easier time learning the basics on easy mode before jumping into something like an MF DOOM song.