I would say the expense is both latency and availability because if one node doesn't ack within the timeframe then you have to drop it from the cluster. Request

I would say the expense is both latency and availability because if one node doesn't ack within the timeframe then you have to drop it from the cluster. Requests that go there would need to be routed elsewhere to avoid being unavailable. If there's a network partition preventing that, then you have partial downtime. If enough nodes fail then you have full downtime across the whole cluster.