I took your advice and went looking for it. I came away with the feeling that some people think they are useful some people thin they are evil. Did you have something specific in mind that you thought packed structures should be avoided for?
Packed structures seem to bring out compiler bugs like not much else does, no idea why but they have caused me many nights of lost sleep over the years to the point where if I see packed structures, there is a bug and the data does not get sent over the network my first instinct is to disable the packing to see if the problem goes away.
That's putting it very mildly.
In a way they were lucky, in this context there was a possible fix, if you're assembling a packet to be sent across the network then you won't be able to solve it that easily.
For more fun like this look up all the caveats around packed structs.