Unreal's unity builds get very annoying as soon as your teams exceeds a handful of developers. You often have code that has builds fine on a machine but doesn't on another developer - it completely depends on what order the UBT picks. This is even more annoying when you have some people on Windows, some on Linux, some on Macs, and they consistently get different results. The fact that most tools (Intellisense, all clang-based tooling, ...) are broken with UE makes automating the detection of missing includes even harder.

We worked around this by introducing a mandatory pre-merge CI stage that constantly does non-unity builds - but it's costly and not something a small company can often afford (a non-unity build of our UE project is ~20min on a Linux runner, and way more on a Windows one. That adds up fast).

Unreal itself hasn't been non-unity buildable for a very long time. In general IMHO Unity builds are a testament to the failure of the C++ standards committee to realise that modules and the building model should be part of the standard to. The "one file is a translation unity" hasn't been adequate for years IMHO - I honestly appreciate how Rust basically imposed cargo as a standard, it was a hard but sane choice.