I would love for save modifiers to return. It would also allow Terminator Armour to be significantly better than Artificer armour, as you could broaden the save range. It could at least be 1+ like you get on the heaviest cavalry in Fantasy.

I have to say a Marine's torso is definitely not the size of a bull, not sure where that one came from.

I'm not going to pretend I've ever researched how well a great big spikey chunk on the end of a monster's tail being swung would penetrate armour. At a guess using lay man 'it seems like it would work that way' physics I would say that because it's being swung wide rather than than being applied with purposeful force behind it like attacking with its limbs would be, it would be worse at pushing the spikes through the armour of something that will give and move away than something that will stay where it is and be caved in. The more stable and immovable something is, the more damage swinging the mace tail at it will do, hence it will do more damage against a tank and cave in a chunk of it, whereas if it hits a guy, he'll give rather than holding steady, so will be thrown.

Correct me if I'm wrong but you're better off wearing more flexible material against a blunt weapon such as a hammer, as it will give and flex around the surface area of the weapon so the force will be dispersed, as opposed to rigid plate which will hold in one place and thus cave in. On the flip side an edged weapon such as a sword or spear will go right through softer, more flexible armour which will bend inwards and make it even easier, but will be more likely to fail or glance off against rigid plate. The difference here, I suppose, is that it isn't just a large blunt object, it's covered in sharp spikes so it isn't just bashing, it's also piercing. I would think that even if the spikes didn't go through you, the force of being hit with such an object with such force would shatter your bones regardless of armour, wouldn't it? I get that blunt weapons are meant to cave in armour, but I imagine the sheer force of a massive spiked object hitting you with whatever strength a Carnifex can put into swinging its tail would probably shatter your bones and render you incapable of fighting.

I imagine power armour would keep you safe enough and a Space Marine has an exceedingly tough skeleton, but anything less (a Guardsman for example) and I reckon the skeleton wouldn't hold up to it, regardless of whether it hits the flak armour or not. Not necessarily dead, but probably not capable of continuing the fight. I could be totally wrong, but that seems like it makes sense.

AP 4 or 5 I would say. Or yeah, just bring in save modifiers.