it is buffered, so is slightly alkaline -- not good for some applications where color dyes are sensitive to an alky environment.

What probably really makes it "archival" is that it is removable with a little more heat.

Without getting into the whole mounting/not mounting discussion (we've done that to death a few times), I think any dry mount tissue would be as "archival" as the next -- discounting the removable aspect of the Buffermount.