I have the Giannini GWNC1/7 which is a classical guitar. I really wanted it to be a good guitar since I'm from Brazil, but while it's not the worst guitar I've played, it leaves a bit to be desired. The frets and the finish are lacking. The headstock cracked, then cracked again right next to it after I fixed the initial crack.
There isn't a lot of repertoire for seven string classical. When playing Villa-Lobos stuff, you might come across a couple of spots where it might make sense to use the low B. But I doubt there is much out there.
My understanding on russian seven string guitars is that they are tuned differently. So I don't know how good they would be for what you want to do with them.
I use my low B mostly when I play jazz on my Giannini. I use it to alternate bass lines when playing chords.