Heroes 5.5 WarGame Edition - Tactical Edition.

That's strange. How can a powerful undead dragon... a lich... be weaker than its living counterparts?
It can actually be stronger; let's remember Viseryon again.

Viseryon, firstly, was well "preserved" (not just bare bones, and certainly not a ghost).

Secondly, weak dragons have always been a hallmark of Necropolis, throughout all eras (except for Heroes 4). But even in Heroes 4, the bone dragon was twice as weak as the black dragon.

Thirdly, let it be a consolation that undead dragons are not alone in their tier; they may be weaker than living dragons, but they are equal in strength to angels and devils (they have a growth rate of 2). I was going to add a cyclops to their company, but it's just too impressive in size. Such a giant as a cyclops would have winged dragons for breakfast and titans for lunch. Although in Heroes 6, cyclops have become significantly weaker, judging by the description; their height doesn't even reach 3 meters.

And in general, this is a characteristic feature of all undead – being weak but numerous.

Overall, yes, but on tiers 4-5 in WGE (vampires and liches), the undead are individually very strong. Vampires are absolute leaders on their tier 4 (in fact, they correspond to tier 5). 1 vampire = 1 knight/griffin (which humans have on tiers 5-6).
Liches are leaders on tier 5 (but they have competitors there – djinns). So, the Necromancer faction didn't come out as underpowered.