TRANSPOSE is not supported and probably won't be until someone submits a patch for it. It's a nasty formula because it has many meanings depending on where it's used and it can be a regular or an array formula.
I updated https://closedxml.codeplex.com/wikipage?title=Evaluating%20Formulas with the list of functions supported.
I updated https://closedxml.codeplex.com/wikipage?title=Evaluating%20Formulas with the list of functions supported.