I've done Grade 8 singing and passed with distinction (ABRSM). I found it equally hard to my violin. I found that the sight singing is a lot more complicated because you have to sing 3 different things on the spot, with words and without. Also, the pieces, for a soprano, are very difficult and you have to sing in different languages. I had to sing 5 pieces. One in German (which was hard to get the pronunciation right), two classical English pieces, a piece from opera/musical theater and a folk song unaccompanied- which made it difficult to pitch right. All of these I had to learn from memory. I think that's a lot harder than having the music in front of you to read from. And scales are so much easier than doing another piece.
However, in violin I am less talented and it's my second instrument so I naturally think it's more difficult. I think that naturally people enjoy singing more because it's in our nature and it doesn't feel like hard work when you practise. However, with an instrument, unless you're naturally talented, it may come as more of a struggle.
But, I think, in terms of grades, that the way that they test you in Grade 8 singing is as equally hard as grade 8 violin or piano or whatever.
Unless you do trinity, which is always easier.