I'm on turn 160 playing a coop vamp campaign in SFO so feel can help you out (and even though I say coop, my friend playing von carnstein immediately went off on an exodus to the badlands and khemri, to I quote "reclaim the motherland" since the vampire curse came from khemri, leaving me to fight empire, brettonia and now dwarves+elves by myself). I am currently embroiled in a massive war against both a united dwarven empire stretching from athel loren to the chaos wastes and a united elf empire safe in their donut. On my side I own the empire and brettonia so have a good power base but am sandwiched between elves and dwarves and fighting on about 7 different fronts. Also dealing with chaos incursions too since that's now my problem. So yeah dealing with lots of highly armored high tier armies.
The problem is going all in on skeletons or zombies and expecting them to win in a head on battle, especially against dwarves. That ain't what they are good for, if you want frontline equivalent to other races, it's graveguard or bust. And in case you didn't know, zombies actually detract from your autoresolve strength (I don't know why it does, but it's something that became increasingly clear when deleting a zombie unit would flip the auto bar from full red to 3/4 yellow in some extreme cases), it's one big factor that forced me to ditch all zombie armies even though I like making armies of thrash work, because I was sick of having to play every one sided battle, my current army comp for ghorst who specializes in zombies is ghorst+1 banshee+7 zombies+ highest tier corpse cart with all the auras+4 hexwraiths+2crypt horrors+4 black coaches (I've learn to love them as line breakers especially in sieges). Still a pretty economic army yet fully capable of taking on even a higher tier dwarf stack 1v1 with no permanent losses. The ghosties melt like snow in hell to anything with magic damage (brettonia's grail knights fucked me over hard) but they are absolute beasts otherwise, routinely getting 300-400 kills in battles (by contrast my elite high tier with all buff zombies are lucky to get a kill count above 10) so a must have against empire or dwarves.
So yeah zombies and skeletons are there to make up the numbers and tie the enemy down in a brawl for your REAL damage units to do the work.
Also another reason why all zombie or skeleton armies are bad: 0 chasing potential. Even when you win a battle you will inflict no casualties on routing units and every enemy hero and lord will escape. This is one reason why I grew to love banshees despite their fragility as they are S tier hero/lord chasers, if they ain't on a flying mount then she will easily make sure no enemy leader leaves alive.
TLDR value of zombies and skeletons comes from how utterly disposable they are and act as the anvil for your other units. You can happily lose your entire frontline of zombies, it's no big deal, you can rez them in the same turn as if nothing happened, but expecting to use nothing but zombies and skellies against dwarves is just asking to be spanked. Some races like empire or dwarves can get away with using nothing but basic units in their army and do great, vampire counts ain't one of those.