Check the Features support page to find out high-level features and the status of their support in different games.
The following items are known to be not working and there is no specific timeline on getting them fixed.
commands requiring an scm variable (e.g. countdown timers). Tracking issue.
commands implicitly loading models or textures (such as widgets) Tracking issue. You can circumvent the issue by preloading needed resources, e.g. by calling them in a .CS script first.
you can't call the game functions that need references to variables to store the result. There is no "take an address of the variable" syntax. As a workaround, if the platform permits (e.g. in San Andreas with CLEO 4.4) you can allocate a memory block and pass its address to the function, then read the result from the memory using Memory.Read.