ToME: the Tales of Maj'Eyal

I know and agree that those difficulties are not fair for the Player but Combat Accuracy talent scaling makes one of the core game mechanics almost useless on Insane/Madness.

On Insane/Madness Combat Accuracy talent on melee/archer rares/bosses is adding to much Accuracy. Almost all this enemies (even low level ones) will have ~100 Accuracy which means that Player Defence is almost useless.
This talent could have diminishing returns from effective talent level or some cap. So in this example 22 effective level could mean something like 100-120 Accuracy. This would give rescaled Accuracy~60 for enemies without lots of Dexterity or Luck. It would mean that not getting hit by pumping defence is still viable strategy for Insane/Madness difficulties.

I didn't think pumping defense worked in normal difficulty anyway. But that does seem Insane.

Plenty of things are useless on Madness.

Enemies being far too strong to use saves/defense against is entirely expected and fine.

Well, actually only two things are really broken on Insane/Madness. Those two things are created by scaling effect from skills:
Accuracy - increased by Combat Accuracy
Physical Power - increased by Weapon Master X

Mindpower or Spellpower will not be as high as Physical Power because there is no common skill that increases those stats. Only some very specialize bosses will get those stats really high.

Mind Save or Spell Save are still moderately useful. Physical Save and Defence are useless.

Plus, Combat Accuracy and Weapon Mastery are among the few talents that scale linearly forever these days. Infinite scaling means most talents are either square-root or asymptotic functions. The Mastery talents are the exception, not the rule, which is strange seeing as just about everyone has them.

Those stats are tiered which means that the stats drop off intrinsically, so the talent doesn't really need to.

But tiers don't practically exists for Accuracy and Physical Power when effective skill level is 20 or more. Linear scaling means that those skills will always give >100 PP and >100 Accuracy after tier rescaling.

When you look on Rare/Bosses stats in tier 2 dungeons it looks like that:

• Melee/archer oriented has 100 Accuracy, 100 PP, 20 MP, 20 SP,
• Mindpower oriented has 30 Accuracy, 20 PP, 60 MP, 20 SP,
• Spellpower oriented has 30 Accuracy, 20 PP, 20 MP, 60 SP,

Difference between Spellpower/Mindpower and Physical power/Accuracy is obvious. This means that something here is not intentional and not working as design.