twocats
You're right that generalization is not good.
Of course there are different factors that makes a piece easy or hard. Remembering the notes is 1 thing but the actual playing we need to understand the hand mechanics such as which fingers are weak/strong, which ones are shorter/longer, hands relaxation especially playing fast pieces. Otherwise some pieces are not very playable.
I've never got into pieces at the ABRSM-10 level. There are 2 popular student pieces including Bach "Prelude in C" from WTC Book 1 & Satie "Gymnopedie 1". After a Google search found both rated between level 5 & 6. Both are challenging in their own way. The Prelude with just repeated chord arpeggios is much more manageable. Once you get the basic idea of holding the 2 bass notes and playng 6 notes on top, the rest is just repetition. Gymnopedie requires big jumps that are similar. After learning to do 1 jump with the help of the sustain pedal to fill in the gap, the other jumps are similar. Don't usually find remembering the notes a big issue.
A few years ago I watched a man play Nocturne Op 9 No 2 on a public piano. He played quite well with the sheet despite some hesitations that were not very noticeable. A piece like this is rated around level 8. How easy would it be to memorize something like this? And there are much longer pieces lasting for 10 minutes or more like a movement out of a Rachmaninov concerto that an average student wouldn't be playing.
A while ago I learned the first movement of Bach Italian Concerto in F just under 4 minutes. Went to a gathering and played it from memory. Good enough to delight some people in the room. 2 years ago I searched online for a piano arrangement of the "Sinfonia" from Bach Christmas Oratorio #2 but nothing satisfactory came up. The original is an ensemble piece with 8 parts played with flutes, strings & oboes. Ended up with my own arrangement and learned to play from memory for 6 minutes in an Andante tempo. A piece that was not originally written for piano and included in an ABRSM /RCM repertoire book doesn't have a rating.