There doesn't need to be exactly one unity translation unit. In Firefox, the build system groups source files by component, and then into "unity" batches containing 5-20 source files each.
If a build spends a lot of time processing code in headers, and source files in the same component are likely to include very similar sets of headers, this kind of configuration gives decent benefits, while not making incremental compilation much more expensive (and possibly cheaper, if you edited a header affecting many files in a single component).
If a build spends a lot of time processing code in headers, and source files in the same component are likely to include very similar sets of headers, this kind of configuration gives decent benefits, while not making incremental compilation much more expensive (and possibly cheaper, if you edited a header affecting many files in a single component).