We have only really seen Death and we saw the denizens of Death able to cast pretty much ANYTHING they wanted. They did not really have restrictions beyond THEME which is a completely surface level restriction (unless you are dealing with fey creatures that are often powered by theme and story). In hard magic systems, magic is practically a science in how it is defined.
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Yeah I think it's just a case of someone not very familiar with fantasy. You cannot really believe these things about WoW otherwise.