I actually quite like the SCAG. To me, that book feels unfinished only insofar as it could have had so much more in it. Xanathar's feels unfinished because much of its content comes across as half-baked.
For all their talk about quality over quantity, I'm just not seeing it here. Xanathar's feels like a random assortment of stuff they felt was "good enough". Much like the monstrous races in Volo's. It felt like the designers just couldn't be bothered spending enough time getting them right, so they just slapped on a warning about them possibly being unbalanced and called it a day. A lot of what's in Xanathar's feels the same way to me.
I'd probably give the SCAG a B+, whereas I'd be inclined to give Xanathar's a B-.