This should be a deal breaker for any serious app. Does the performance hit of safe mode negate all other advantages of MongoDB?

That's most people's findings. If your dataset can fit in ram [1] and you don't care about your data being safe then there might be an argument for MongoDB. Once you care about your data, things like Voldemort, Riak, and Cassandra will eat Mongo's lunch on speed.

[1] But as Artur Bergman so eloquently points out, if your data can fit in ram, just use a native data-structure (http://youtu.be/oebqlzblfyo?t=13m35s)

Do you have a citation to back up the claim that you shouldn't use MongoDB for serious apps?

We have done billions of ops with Mongo and have never lost any data.