Under Steem each producer is responsible for a 3 second window of time... under the proposed changes EOS.IO would have each producer responsible for a 6 second window of time during which 12 blocks would be produced. As those 12 blocks are produced the other 20 producers also sign them and make them irreversible (after 2-3 blocks, 1-1.5sec).
So as described above, the biggest issue is a producer going down (not producing) in which case there would be 6 seconds (12 missed blocks). This is the same with Steem, a single producer going down creates a 6 second gap with no confirmed transactions.