On one hand,in systems where processes and threads are considered as similar entities, some of the operating system code could be simplified.
A scheduler, for instance, can consider the different processes and threads in equal footing without requiring special code to examine the threads associated with a process during every scheduling step.
On the other hand, this uniformity could make it harder to impose process-wide resource constraints in a direct manner. Instead, some extra complexity is required to identify which threads correspond to which process and perform the relevant accounting tasks.