In the event of a single server crash, Mongo may need to be restored from backup, or another master. CouchDB handles this much better, and can just restart where it left off. But in the worst possible single-server scenario, there's smoke coming out of your server and the hard drive is toast. CouchDB and MongoDB perform about the same - if you had replication or a recent backup, you are fine. If you didn't, you are f*cked.
CouchDB is better for small servers which crash a lot and can't use replication. You can run it on a mobile phone. Mongo is better for logging (where you need fast writes, and don't care about a couple of lost records). Most uses will be somewhere in between, and you need to know the limitations of the stack, so you can work around them.