Seems to me that instead of the EFB being a problem as such, we just use it in a different way to the old paper documents. As others have mentioned above, unless on approach (where I keep the efb in the cradle - exactly the same place the charts used to be clipped, coincidentally) I have the efb/ipad on the table in front of me so my neck is straight. I think it's easy to be 'lazy' and try and use it while is in the cradle, but you never used to do that with any paper, so why start with the efb?
I will add though that i found over the covid period any neck pain/stiffness i had while flying went away - i suspect a mixture of awkwardly looking up and sideways to the overhead or turning 180deg to see the door cam wasn't good at all!