Disadvantages
Depending on the design & architecture of the processor, simultaneous multithreading can improve performance if any of the shared resources are limiting bottlenecks for the performance. In fact, some applications run faster when simultaneous multithreading is enabled. An example of this is the Intel Prescott Pentium 4 HT cpu. Critics argue that it is a considerable burden to put on software developers that they have to test whether simultaneous multithreading is good or bad for their application in various situations and insert extra logic to turn it off if it decreases performance. Current operating systems lack convenient API calls for this purpose and for preventing processes with different priority from taking resources from each other.
There is also a security concern with certain simultaneous multithreading implementations. Intel's hyperthreading implementation has a vulnerability through which it is possible for one application to steal a cryptographic key from another application running in the same processor by monitoring its cache use.
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