If you are manipulating the variables in a physically consistent system, the units help a lot . but a lot of property models are … blackboxes, 
. , non consistent equations, magical constants. Even the authors of said models didnt add any units to their models, as those models are the result of an optimization algorithm.
Returning units from the property functions can be be solved with adding one uconvert(preferred.unit,val) to each value passed to each model in each implementation, and a second uconvert from the model’s output models to the desired output units. so its doable. it just transfers the responsability of converting from the state to who implements the model. i decided that the state has to have the responsability of managing the unit conversions.