"Fewer" people? Isn't there just one person complaining?
Actually it is remotely possible that the Doomsday Machine could've been armored in actual neutronium, if the neutronium could be encased in some kind of material strong enough to resist its degeneracy pressure -- similarly to how Robert L. Forward proposed that electron-degenerate matter (i.e. matter of white-dwarf density) could be encased in synthetic diamond (see my discussion of this in Ex Machina
). The DM actually does look as if it's coated in some kind of shiny, smooth material (which is actually plastic sheeting on the original miniature, but it looks really neat if you don't examine it too closely). It's a reach, but it's within the realm of suspension of disbelief, given the super-advanced technology in play.
It's when you get to the later uses of "neutronium" in DS9 and VGR, as an armor material on doors and stuff, that it becomes implausible, because of the sheer weight and density of the substance. That's where it's handy to think of it as a shorthand for some kind of material that isn't literally neutronium.
Hmm... now, Memory Alpha makes the claim that "Since 1942, it has been used in various science fiction material to refer to a super-dense alloy or a trans-uranic element on the periodic table."
However, the OED's SF terminology citations site
doesn't quite agree with that. There's a reference to a "neutronium hull" in a 1931 Jack Williamson story, but most of the references cited there (except the ones from Trek novels) are referring to the actual neutron-star kind of neutronium either literally or as a metaphor. (The 1942 Hal Clement reference to cities of neutronium is in a story about beings that live in the core of the Sun, so it's certainly not about a material that could survive in M-class conditions.)