The most significant risk in a unity build using standard include guards (#ifndef HEADER_H) is Macro Name Collision.
In a standard build, source files are compiled individually. If LibraryA/utils.h and LibraryB/utils.h both happen to use the include guard UTILS_H, it rarely causes issues because they are usually compiled in separate translation units.
However, in a Unity Build, dozens or hundreds of files are combined into a single translation unit.
The preprocessor reads LibraryA/utils.h, defines UTILS_H, and includes the content.
Later in the same file, the preprocessor reaches LibraryB/utils.h.
It sees that UTILS_H is already defined.
It silently skips the content of LibraryB/utils.h.
This results in confusing "incomplete type" or "undefined symbol" errors because the compiler literally ignored the second header file.
**#pragma once prevents this.** It relies on the file's unique identity (inode, file path) rather than a user-defined name. Even if two files are named utils.h, the compiler knows they are different files and will include both.