What are the disadvantages of using threads?

Because threads have some of the properties of processes, they are sometimes called lightweight processes. In a process, threads allow multiple executions of streams. This is why thread needs its own stack. An operating system that has thread facility, the basic unit of CPU utilization is a thread.

There is a lack of coordination between threads and operating system kernel. Therefore, process as whole gets one time slice irrespect of whether process has one thread or 100 threads within. It is up to each thread to relinquish control to other threads.

User-level threads requires non-blocking systems call i.e., a multithreaded kernel. Otherwise, entire process will blocked in the kernel, even if there are runable threads left in the processes. For example, if one thread causes a page fault, the process blocks.