memcached is only a caching mechanism. It shouldn't be used to store information that you cannot otherwise afford to lose and then load from a different location.

There is no security built into the memcached protocol. At a minimum you should make sure that the servers running memcached are only accessible from inside your network, and that the network ports being used are blocked (using a firewall or similar). If the information on the memcached servers that is being stored is any sensitive, then encrypt the information before storing it in memcached.

memcached does not provide any sort of failover. Because there is no communication between different memcached instances. If an instance fails, your application must capable of removing it from the list, reloading the data and then writing data to another memcached instance.

Latency between the clients and the memcached can be a problem if you are using different physical machines for these tasks. If you find that the latency is a problem, move the memcached instances to be on the clients.

Key length is determined by the memcached server. The default maximum key size is 250 bytes.

Using a single memcached instance, especially for multiple clients, is generally a bad idea as it introduces a single point of failure. Instead provide at least two memcached instances so that a failure can be handled appropriately. If possible, you should create as many memcached nodes as possible. When adding and removing memcached instances from a pool, the hashing and distribution of key/value pairs may be affected. For information on how to avoid problems, see Section 14.5.2.4, “memcached Hashing/Distribution Types”.