To a certain extent an exam is as hard as one makes it. One could go into a diploma exam with the attitude of getting it the easiest way possible and could probably pick a complete programme of pieces that had been on various grade 8 syllabuses. Similarly one could go into an exam and play the pieces one likes, these happening by chance to be all amongst the hardest, and do a lot more work for the same thing. I think it's obvious from which approach one would benefit the most (even if the pieces one liked weren't the hardest), but unfortunately the emphais seems to be shifting, for some people at least, to a point where getting the letters is the important outcome of any exam; not what one learns on the way (just count the number of people in a university lecture theatre when the lecturer has said 'this won't be on the exam'!).
I think deciding the relative difficulty of exams based solely on the repertoire set is a little dangerous (especially when with diplomas where the repertoire lists are so vast). People often say diploma X is harder than diploma Y because the repertoire is harder, but it depends whether one picks from the easy or hard end of the list here's a programme which 'proves' that ATCL is harder than dipABRSM (or some would say as hard as LRSM, all pieces being on the ATCL and LRSM syllabuses for piano):
Bach - Prelude and Fugue in C#, BWV 848
Mozart - Sonata in C minor, K457
Chopin - Etude in F Op. 10 No. 8
Faure - Nocturne No. 4 in Eb
Ginastera - Danza del gaucho matrero (no. III from Danzas Argentinas)
And here's one that 'proves' dipABRSM is harder than ATCL (or some would say is as hard as LTCL, all pieces being on the dipABRSM and LTCL syllabuses for piano):
Ravel - Sonatine
Mozart - Piano Sonata in D, K284
Szymanowski - Etude 3 Op. 4
With the same logic I could probably 'prove' that neither diploma is harder than grade 8 and, in instruments like french horn, which have less repertoire dipABRSM = LRSM = FRSM (I did actually speak to someone here (hgirl) whose dipABRSM programme, with which she passed incidentally, could have also been used at LRSM and FRSM). What really makes the diplomas hard is the standard of playing required, 50% of the people who enter the AB's diplomas, who presumably can play the pieces in their mind or they wouldn't enter, do not pass. What makes it difficult is not so much what one has to play; but
how one has to play it. Yes, to a certain extent the repertoire difficulty does make a difference (I'm not going to sit here claiming that Chopin Etudes are easy!) but it's not quite as significant when the playing standard is considered. I'd guess it's easier to play an LRSM programme to dipABRSM standard than a dipABRSM programme to LRSM standard (or maybe more clearly a dipABRSM programme to grade 8 standard than a grade 8 programme to dipABRSM standard).
P.S. MrB this isn't a complaint at you, I know you have experience of the standard of a lot of these things (I wouldn't argue with you on the matter

), it's just a general observation that many people use piece standard as an exam difficulty metric.