The #1 problem with this movie
is that at no time after the mind-transfer do we get the impression that it is the 68-yr-old Damien now inside the body of Mark. Even when he's on the anti-rejection medication he talks and thinks like Mark, not like the cranky old businessman who should be in there.
The #2 problem: That ridiculously cheesy contraption with special effects fit for a 60s Star Trek episode that does the mind transfer. And if metal makes it not work properly why doesn't all the iron we have in our blood along with all the other trace metals in our body have any effect on it?