As long as you use Membase buckets (in Membase there are memcached and Membase buckets) then everything will be persisted to disk. If the items on disk are greater than the number of item that can fit into memory then only the most recently used items will be held in memory.
Also, it is recommended that your working set fits into memory. If your working set is greater than can fit into memory then Membase will begin to take a performance hit.
Membase is also very safe. It runs in many mission-critical production systems. Zynga for example uses it to power Farmville.