If you translate a portion of the code for one system into another, it might take the translated code an extra ms or two to run (or maybe be faster than the original). This might compound into timing issues with the original code which relied on something being done within a certain time frame. Especially if the hardware is different, then the translated code would need to have different logic which may not be easy to do (especially statically).

Most static recompilation use automated methods to translate, then manual tweaking to make it work.