Setheum’s finality protocol for consensus is the very healthy GRANDPA consensus algorithm. GRANDPA (GHOST-based Recursive Ancestor Deriving Prefix Agreement) finalizes batches of blocks based on availability and validity checks that happen as the proposed chain grows. The time to finality is expected to be very fast.
Setheum is able to provide stronger guarantees with fewer validators per shard. Setheum achieves this by making validators distribute an erasure coding to all validators in the system, such that anyone - not only the shard's validators - can reconstruct a block and test its validity. The random validator assignments and secondary checks performed by randomly selected validators make it impossible for the small set of validators to collude.