Huh? Remember: Siege is immune to poison. (that is, if immunity works. But it will one day for sure, so we can pretend...)
They are actually better against assasins than their adept / mage counterparts (especially after poisoned blades are applied).
Especially the dwarven trebuchets... (which are the only ones likely to be built often. At least in my experience.)
+ repairing them is easier even for Khazad than doing so for Living units (let alone Pyres). They "just" need Alteration.
+ they come way earlier than Assasins. So some of those are likely to allready have some promotions making them even more formidable oponents... Also they are rather cheap in comparison. Nothing i whould like to waste my Assasins on either way. (with such a stack of cats / trebs i whould need a sizable force of Assasins on my own (harassing them a bit with some ghosts which are my usual assasins might work in both cases, but still not the best targets... and just some of them won't destroy a whole stack on their own and reliably.). If i don't destroy the vast majority of the stack mine will be toast just as well. There like with pyres i whould rather throw strong things at them one by one and use spells to hamper them / slowly wear them down. A Ranger / Satyr or something like that sounds well for such a thing... As do mounted units.)
That doesn't make them compare to pyre zombies naturally (trebs do come somewhat close though. At least with support from some though, bearded short-guys. And for actually taking cites out of AI hands trebs might even do a tad bit better if supported.).
But both a stack of trebuchets/cats with some support and a stack of Pyre Zombies with some support can be terrifying if they do appear early enough. (And in emperor + its not unlikely to see them rather early. On emperor, such stacks are not completely impossible for midgame if such a civ has some sucess at establishing itslef. At deity they should be a common sight. Adepts summoning Skelletons also are rather nasty and are a common sight on higher difficulties as well.)
Also sieges are immune to destroy undead. Might not seem like much at all (and won't help early unless you are Elhoim). But it will do much more so with the implementation of explorable lairs. With which the spell will become way more popular for sure...
Honestly, they are comparable in a way in both strengths, weaknesses and counters. Pyres are "just" much more stable on their own + can kill units on their own. But then the Sheaim have few other martial things to back them up. (and going for them detracts from going for other things that make the Sheaim powerful.)
And Sheaim are not a late civ anymore since they got death-mana at start.
Skelleton-swarm (followed up by spectres and wraiths naturally.) ftw... Free easily replacable champions (with one first strike as well? Don't know for sure if the dance effects them as nonliving ones) do kick butt at early Tier 2 (earlier usually than copper Zombies...). With enough adepts they even handle Tier 3 more or less solidly.
The only point they really lack now is midgame but thats allright. They need at least some kind of drawback... They are top-tier nontheless.
To add some "solution" to the mix: Units with fire-resistance (ideally immunity of course
) might help.
But those might be hard to come by... And they whould still need to win vs. the zombies "martial" part...
Also with the new requirements for Guardsmen that might turn out to be somewhat of an option as well...