the all history of v8 engines, valve cover leaks have been common, primarily leaking right on the mainfold and smoking off, with some coming out the ends and running down in different places. i haven't studied the 5.4 as close as some of the older engines, but if it is the HG, i would assume its the high pressure gallery that delivers oil to the head (as opposed to where oil drips back down). if yes, the big fear is that it may suddenly blow out, seeing that its already weakened, and cause a sudden loss of oil, which you may not notice until its too late.
if its just valve cover, i've seen them get bad enough that you stop the car and notice smoke all around you because it was leaking so much, but had no other ill effects.