Final answer:
Scheduler activations are less portable due to platform dependence, requiring specific support from the operating system kernel, which can vary widely between systems making portability difficult.
Step-by-step explanation:
The scheduler activations approach has the disadvantage of being less portable than either user-level or kernel-level threads primarily because of platform dependence. Scheduler activations provide a mechanism for the kernel to inform a user-level threads manager about certain events, such as when it's necessary to create or destroy threads or when a thread blocks. However, implementing scheduler activations requires specific support from the operating system kernel. If the underlying OS does not support scheduler activations, then this approach cannot be employed.
The problem is that different operating systems might have different interfaces and levels of support for this functionality, making it difficult to write portable thread libraries that use scheduler activations. In comparison, user-level threads can be implemented on top of almost any OS, and kernel-level threads are directly supported by the operating system itself. Scheduler activations require a mix of support from both the user thread library and the kernel, and any variation in the kernel's API or internal mechanisms can cause issues with portability.