Some criticisms of multithreading include:

Multiple threads can interfere with each other when sharing hardware resources such as caches or translation lookaside buffers (TLBs).

Execution times of a single thread are not improved but can be degraded, even when only one thread is executing. This is due to slower frequencies and/or additional pipeline stages that are necessary to accommodate thread-switching hardware.

Hardware support for multithreading is more visible to software, thus requiring more changes to both application programs and operating systems than multiprocessing.