The SFF-8639 ia a faster interface designed primarily for SSD products in a 2½” form factor. More recently it has been rebadged as U.2 to compete with M.2 which is more constrained physically. Both U.2 and M.2 can use up to 4 PCI Express lanes.
There are more pins which increase the bandwidth considerably to nearly 4GB/s which is now close to the fastest SSD products in the market. SFF-8639 came from server solutions where extreme bandwidth is needed.
The new cable is heavier and more expensive. It includes power which is similar to the SATA and SAS formats. The cable needs 2 SATA Express ports in addition to power.
The Intel 750 2½” form factor has the SFF-8639 interface but it is 15mm thick which limits where it can be installed. Our old Corsair 300R can handle them easily with its removable trays.
We have seen U.2 SSD mounted on a PCI Express x4 card, M.2 slots and every other interface imaginable are also widely available.
Intel is backing the new faster PCI Express 3.0 interface as the solution for the fastest SSD products. AMD Zen will also support the new higher performance options.
The Samsung 16TB SSD uses SAS which has much less total bandwidth compared to SFF-8649. The PM1633a is a 2½” form factor SSD but it’s 15mm thick,
Seagate has a 60TB SSD which also uses SAS and it using the larger 3½” form factor. Again the SFF-8649 would have afforded much more bandwidth.
As the image shows, the SATA uses fewer pins for signalling compared to SAS. SAS is equivalent to dual SATA ports and cables exist to split SAS into a pair of SATA cables. SFF-8639 however has 4 PCI Express lanes which is double the bandwidth of the consumer M.2 port which only uses 2 PCI Express lanes.