Nigd3,
Full colour HUDs are available, thereby replicating a head down PFD in that respect. In terms of criticality, I take it you are talking about what happens if the HUD fails? Pretty standard for a HUD failure to cause a PFD to come up on the appropriate HDD so that you just end up in a 'normal' situation. As I said HUDs have been certified as a PFD so I imagine these problems have been addressed.
Northbeach,
I know what you mean with regard to HUD masking and as an annoyance on the ground. I found that in the air it just takes small movements of the head to effectively "look behind" the HUD symbology. On the ground it is usual to just push the HUD forward. This effectively turns the HUD off, brings up a PFD on the HDD, and allows a clear view forward. The normal stowed position of the HUD is found by pulling the HUD towards you, causing it to rotate upwards and finish flush with the ceiling of the cockpit. I am guessing that the HUD you use doesnt move? Obviously different equipment will need different techniques, so what works on one wont on another.
As an aside there are a few other problems with using a HUD, as Northbeach points out. Very bright lights on approach can cause parts of the HUD to become difficult to view for example.
I think that with some of the technology out there the HUD is going to become pretty commonplace. The G550 for example already uses a HUD to display imagery from a forward mounted camera for low vis ops. As such it is going to be in effect the PFD, so why isnt this taken into account? Just seems like a waste to have all the PFD info duplicated.