Mine is OOO for Out Of Office. I always misread it in my head like a ghost and it takes me a few seconds to process. It also doesn't translate to speech—you have to say the whole thing.
Interested to see if others have similar acronyms they beef with.
Saving a few decieconds is the most upside they can possibly have.
Potential downsides: the reader has no comprehension, has to spend several full seconds or minutes asking the meaning, has to spend extra time mentally processing the text cos it's coded.