Record+playback in a complex, interactive environment like a game pretty much has to be done in situ and not as part of a traditional unit test. The recording is intended to help QA set up a saved game and give a repeatable scenario to engineering for bug fixing.
Unit testing of core components IS useful, but only in the context of ensuring that the engine itself is functioning properly.
Examples of engine unit tests would be to load a static scene and compare a render versus renders in the past, or (more on topic) to load a player and feed it recorded inputs to test whether it ends up at the same coordinates as previous runs.
There are other, non-deterministic tests that are very useful in online games. The best example is load testing with a large number of headless clients fed from a pool of recorded inputs, this would catch networking problems and help the engineers optimize.
And there are failure tests where all sorts of bad juju is thrown at the engine to help flush out problems, including malformed inputs as part of the test.
In summary, record+playback is useful but in constrained ways that supplement living people instead of replacing them.