tl;dr - Difficulty rises exponentially, so rank SS is incomparably harder than rank S, same for rank S to rank A, see first 4 videos of this playlist.
There's a many fl variations so I made letter rankings to explain difficulty to other spinners. Estimates comparing different tricks listed later (based on my own experience).
Pen spinning is hard, because doing a combo of multiple links/tricks generally has difficulty rising exponentially, i.e. combo of link A > link B > link C with 1/3 chance of doing each link, chance of completing is (1/3)^3 = 1/27. It's also why you need many drafts to get decent execution.
In first learning cont, chance of
progressing to next one (going from bust x 3 to bust x 4) is low, maybe 1/10 or worse. For someone who can do 10 busts,
the difference in bust x 5 and x 9 is big. As you get
better, this chance improves, so the difficulty rise becomes more linear (someone who can do bust x 200+ finds bust x 20
and bust x 40 easy), because chance of doing next bust
successfully is high.
difficulty is related to how many 'points of failure' that can occur when doing the trick. For bust
and spread, there is one motion/point that can lead to
failure, and that point has big margin of error. Fl ia ss, fl ma ring ss still only have
one point, but that point has smaller margin of error. Mirr pp rev pinky ss has 3 or 4 points of failure, each of which has very small margin of error.
There's time when I felt trick getting from 'exponential' to 'linear' chance as I improved. For tricks with one point of failure (bust or even a harder trick like fl ma ring ss), that happened around x 10-12, so doubling from x 12 to 20+ was a lot easier than doubling from x 5 to 10. For a hard trick with multiple points - mirr pp rev pinky ss I've raised to x 17, I still don't feel this.
So, in comparing trick which keeps rising exponentially with one that has become linear, the exponentially rising one is incomparably harder at higher numbers. Mirr pp rev pinky ss x 5 is comparable to fl ma ring ss x 30 in difficulty, so mirr pp rev pinky ss x 15 is comparable to fl ma ring x 70-100, right? This isn't true because mirr pp rev pinky ss rises as exponential from 5 to 10 and 10 to 15, whereas fl ma ring ss rises linearly after x 30-40.
Roughly: fl ma ring ss x 5 similar to fl ia ss x 15-20, fl ma ring ss x 10-15 similar to fl ia ss x 50-70.
Rank A fl ma ring ss: decent spinner with foundation in fl trains daily 30-60 min for 6-12 months can expect to get about x 10. Rank S mirr pp rev pinky ss x 5 is comparable to fl ma ring ss x 25-35.
Rank SS: difficulty of mirr pp rev pinky ss x 10 (x 9 to 13) in sequence of < 3 seconds (2 or 3 tricks). Rank S+ is fair bit harder than S but not quite SS, so it's quite variable. Logically, rank SS sequence of ~ 3 seconds makes fl ma ring ss into trash, because it has difficulty of 100+ fl ma ring ss in 3 seconds.
The benchmark rank SS is mirr pp rev pinky ss (+0.5 spin pinky ss), x 2 is harder than mirr pp rev pinky ss x 10, and x 3 is comparable to mirr pp rev pinky ss x 17-20.
SSS will have the difficulty of mirr pp rev pinky ss (+0.5 spin pinky ss) x 6-8 in 3 seconds. I'm not anywhere near this yet, but I hope that in future, I can use rank SS as finishers, put multiple SS in one fl combo, reach rank SSS and higher, so I'll keep practising.