The new bup version can do pruning, FWIW (haven't tried to see how hairy it actually is).
bup does have a huge advantage of deduplicating multiple machine images that can backup simulatenously. With Borg, there is a lock held on the repository (only one machine at a time), and if multiple machines do use the same repository, they will need to download the indices (Slow compared to bup's bloom filter), or use sshfs etc (slow)
borg also has internal encryption. If only borg adopts bup's bloom filters and concurrent access, it will only have advantages...
bup does have a huge advantage of deduplicating multiple machine images that can backup simulatenously. With Borg, there is a lock held on the repository (only one machine at a time), and if multiple machines do use the same repository, they will need to download the indices (Slow compared to bup's bloom filter), or use sshfs etc (slow)
borg also has internal encryption. If only borg adopts bup's bloom filters and concurrent access, it will only have advantages...