I suspect that most passengers would prefer loco hauled trains to DMUs, other factors being equal.
However the main problems with voyagers are not the underfloor engines, but are
1)too short
2)cramped
3)toilets stink.
4) generally nasty !
The first problem could been easily solved by building more of them, or keeping some other stock in addition.
The cramped interior is largely due to the tapered body sides, though a better interior fit out might help a bit.
The toilet stinks to me smells to me like incompetent design or manufacture, toilets should be a mature technology that are well understood.
"generally nasty" is rather subjective, some people like them.
But I suspect that the dissatisfied outnumber the satisfied. It seems a very poor design to make the windows so small, and not aligned with most of the seats for example. And still worse to provide windows for luggage racks but not for seats.