Here’s another case where mastering yourself is a nice deal. You have full access to all the tracks as well as the effects on each track. A mastering engineer has to use tools such as multiband compressors and fine-tuned EQ curves to balance out the levels and the frequencies. While you have access to all those elements and you could address problems in the mix itself.
There’s a new trend of stem-based mastering, where the mastering engineer will get stems of the various components of the mix, such as the drums, the bass, and so on. However, even with this, they have a limited way of addressing and fixing problems, not that they could do miracles with good ears and tools, but it’s still a struggle compared with going in and addressing problems in the mix itself.
This is why many of us do a combinational mix and mastering at the same time, build the final product organically. I do have the mastering tools on since the early days of the track, and I could go in and adjust settings while doing the mix, or figure out what parts of the mix that needs to be eq:ed in good time before the final product is done.
With such combinational mixing and mastering the final product will bubble up after a while, and in some cases I didn’t even bother to do a final mastering step, as mastering was done all along working with the song.