The difference does seem to be fairly arbitrary at this point. I'd rather they revised the rules for walkers and monstrous creatures or merged them somehow, but that's bias from someone that remembers the Eldar Dreadnought long before it became a "Wraithlord" (don't get me wrong, I think it works well as a Monstrous creature.) Having vehicles the count as creatures (Riptide) and creatures that count as vehicles (Forgefiend) just seems to be muddying the waters considerably.
Having monstrous creatures as a separate category made sense when we were considering Tyranids, but a Carnifex functioned pretty well when they were just classified as really, really big infantry. The introduction of the Monstrous Creature unit type is just convenient shorthand for a collection of special rules they now have.
The big issue I see with this is it is creating artificial differences between units that don't really need to be there. It won't be long before regular Space Marines have a monstrous creature, which feels wrong to me. They already have a man-in-a-can. Dreaming up a small Dreadnought or a really big Terminator would just be silly from my point of view.
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