Undead are for from indestructible in the new version of the game. First of all don't use blunt weapons against undead unless you are in a reanimating biome or for some reason you haven't bothered to flush out and take care of any necromancers that may have shown up with them. You want to use picks and other powerful edged weapons since it easier to chop a head or other vital body part off than it is to pulp it.
Undead now rank as one of the more powerful enemies dwarves can face, although they are still light years from being at the same threat level as the HFS. A single inorganic HFS with serious deadly dust can melt hundreds of legendary melee dwarves in a strait fight. Having said this, the undead are still VERY powerful opponents and should be treated with the utmost respect. Standard zombies are similiar to husks now in difficulty and should be treated the same way you would treat husks.
I think the standard cannon fodder zombies are bit to powerful and could use some tweaks, mostly they shouldn't be super agile and super strong like some of the more elite undead units like husks and vampires are but they should definitely retain their durability as this is very much in line with zombie lore. Classic zombies shrug off normal wounds but are slow and clumsy with above average strength.
Edit: Since my old arena testing on the undead was done back when combat was broken I decided to run a few quick tests with 10 steel clad legendary axe lords vs standard unarmed zombies. In a 10 v 10 battle against standard dwarven zombies the axe lords completely destroyed them without losing a single member. I ran this test twice with the same result. I then decided to keep throwing waves of 10 zombies at the same squad of axe lords and this where the zombies tireless undead nature began to shine. The axe lords slaughtered another wave of 10 and lost a single member but then the 3rd wave of zombies took them out as the living dwarves began to tire and collapse from the pain of their injuries.
So basically the results indicated that a well armed and very well trained dwarf can easily take out standard zombies in a one on one fight but in instances where the undead outnumber the dwarves caution must be taken to rest and heal the dwarves between skirmishes. If you run your squad of ten legendary axe dwarves strait into a mob of 30 or 40 zombies they aren't going to win. The zombies are significant enemies and can't be treated like common goblin trash.