From my experience... which is fairly limited:
Violin ABRSM and violin Trinity look around the same, though of course you can avoid scales. Scottish board fiddle grades look technically way easier... but while remaining in first position up to grade 5, factors like speed, ornamentation, variation of the melody etc. come into play.
Viola ABRSM grade 8 looks around the technical standard of (in my opinion) violin grade 6. That is primarily because the higher positions aren't used so much by violists, while there is likely to be greater emphasis on tonal quality and expression. Having tried to play the grade 8 viola music, they are actually of a similar standard with a lot of double stopping required - it just looks easier because it's not high.
ABRSM classical piano and Rockschool contemporary piano are actually more or less on a par. That is to say, I find the Rockschool grade 8 rather technically challenging. In the early grades, however, the Rockschool pieces are very formulaic and rely heavily on patterns, perhaps because many students will be learning them by ear? There are more scales required for Rockschool if you take the full exam (rather than performance certificate). I haven't seen Trinity but I've been using LCM jazz material. The LCM grade pieces for jazz piano are marginally easier than ABRSM grades, but the jazz grades require improvisational skills even at grade 1 and by grade 6 it's like "here's a lead sheet, play!" They are assessing a totally different subject.
Flute and clarinet grade 1 are way easier to get to than violin grade 1, even though the music may look of a similar standard. I think the clarinet grade 2, however, is a big jump up while violin grade 2 is not a huge step up from grade 1. All the instruments have their big jumps at different places.
Singing is... well... singing. For a start, you'll only ever have one note at a time. You have to sing the pieces from memory. And you will be marked on intonation (not an issue on guitar), breath control/technique, vocal quality and expression. Nerves can seriously affect intonation. But pieces can look easier than they are, and perhaps be easy to play/sing the notes but hard to sing/play really well.
Nationally, all the exam boards seem to be treated the same by Ofqual, if that helps, but they assess slightly different skills, I think.