So the choice Is between an electric pump that may fail and a mechanical one that may leak. Seems like there are many more stories of electric pump failures on the forums. Perhaps a combination that uses the mechanical one as primary to draw through the electric and using the electric for a backup and for system purging when changing the filters.
Electric pumps are highly reliable--or at least the highly reliable ones are! For example, the Facet Gold Flo are quite good. Plus, I don't think electric fuel pump failures are all that common as a rule, and I read a few different sailing fora daily. Sure, it happens, but not that often.
Also, the form of your statement suggests that mechanical ones don't fail but merely leak. They do indeed fail as well as leak. And one needs to consider also one of the failure modes when they do go. If the electric pump craps out, your engine will die. If the mechanical one has the diaphragm leak, it dumps diesel fuel into your oil pan. I'd surely choose the former over the latter any day.
Consider also that if one is overly worried about the very unlikely event that a decent electric pump would fail, one could always carry a spare that could be easily swapped in. Set it up with quick disconnect connectors both for the hoses and for the wiring and you could switch it out in a few minutes. Swapping out a mechanical pump would almost certainly be a bigger job.
In any event, many (most?) diesel engines only have electric pumps, including the larger Universal models, and they all seem to do just fine. (My previous boat had an M25XP, with an electric fuel pump that was still going strong when I sold the boat.)
As for going with a combination: This may be a bad idea. Depending on the pump pressure, it *might* actually effect damage to the mechanical pump's diaphragm. In any case, there's no reason to do it because an electric pump is just fine, and if one really wants redundancy, carrying a spare electric pump will do the trick.