Hmmm... HUDs are fantastic (almost all my flying done on HUD aircraft) but they do come with significant training burden, as well as significant human factors issues.
Info overload is generally the biggest problem - you can ultimately display every bit of info from the MFD/FMS on the HUD if you want depending on manufacturer and size.
As for their benefits, once you go HUD, you will never go back! They are the bees knees. Fantastic for IF, especially IF transition. And they make their money in low viz (both T/O and LDG).
I struggle to see how they could be cheap enough to be viable in GA; especially with the training overheads.