Not all HUDs are created equal.
We talk of using the HUD to perform certain tasks, to provide information, and to make life simpler. Joe Bill and I refer primarily to the F-16 HUD. However, I will comment on HUDs in general because its important to note that specifications and standards have not been rigorously applied. So what you have are HUD capabilities and characteristics that vary greatly between aircraft types. Originally, the HUD was not envisioned to fulfill a requirement for instrument flight indication. The classic control, performance, and navigation instruments essentially satisfied those needs. The HUD was designed as an air-to-air and/or air-to-ground weapon delivery reference. As such, symbology, layout, fields of view, and information sources varied greatly. But it soon became apparent that the flight path marker or velocity vector information, if accurately displayed, provided both control and performance indication without a requirement to interpret and integrate separate indications. As a result, pilots flying airplanes with more capable HUDs (the A-7D or A-7E, for instance) soon included the flight path marker in their instrument scan. Many pilots progressed further to making the flight path marker/pitch ladder combination the hub of their scan (instead of the ADI) and now included the ADI as just another instrument in the scan. This transition was crucial since it affected the pilots control strategy. No longer did he have to control one or two indications and then interpret five or six others in estimating the airplanes performance state. Now he could use the same indication to control the main performance parameter, the flight path.
Since HUDs have not been integrated into the aircraft to provide an instrument flight reference, each HUD has to be independently assessed to determine whether (or to what extent) it can be used for instrument flying. The F-16, particularly the C/D model, has the necessary HUD capabilities for safe and precise instrument flight. The HUD is reliable, provides failure indications, contains accurate and usable symbology, and (in the C/D) has a good field of view. And the so-called primary flight instruments provide both an adequate cross check (should you feel the need) and system redundancy in case a failure occurs in the HUD, in its display generator, or in an information source (that is, in the central air data computer).
So when anyone addresses the topic of the HUD or using HUDs, he must qualify the specific capability to which he is referring. Some level of standardization is required in HUDs. But in the meantime, individual HUDs can be independently assessed as to their instrument flight reference capability.