A lot of people are still thinking of the Shadowlands as a spiritual realm, analogous to the afterlife realms of most religions and so forth. In terms of the WoW cosmology, the Shadowlands (like the Twisting Nether) is actually a real and physical place - basically a different plane akin to the elemental planes like the Firelands or Deepholm, and while it has different rules it is functionally material. As for Maldraxxus and Necromancy, it seems to function pretty much the same as it does in our (the PC's) universe, except with the caveat that destruction of the soul is permanent in the Shadowlands, as there is nowhere else for souls to go unlike in our universe (where they shift to the Shadowlands after a time). Construct bodies appear to protect and preserve the soul to a degree, making it possible for them to be transferred from construct to construct. What's more confusing to me is individuals like Draka, Vashj, or Mograine - who appear to be themselves, more or less how they were in life, and not really constructs like the various abominations or other beings in Maldraxxus who look like more typical undead.
Necromancy in the Shadowlands is more about giving an entity additional armor and/or weaponry via housing them in a weapons platform as opposed to just restoring them to physicality after death. To the same extent, you could say the Kyrians are themselves Necromantic constructs of a sort, in that they are new bodies or forms created to house the souls of those judged to Bastion by the Arbiter. Same with Venthyr who were previously mortals. I would've probably not reused the term "Necromancy" for what the Maldraxxi do in favor of another term like Soulsmithing or Spiritshaping - which doesn't have the same connotations as the Necromancy we're used to and more accurately describes what the Maldraxxi are actually doing with their constructs.