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I remember that in past editions Durability and Versatility didn't mix, but I'm not sure about 3ed.
Since Durability is now an optional talent, is it possible for a Human Wizard to take the Durability of another discipline with versatility instead of his own?

ED3, PG, pg 242-243.
As a rule, durability is fixed. There is however an optional rule, which allows for the realigning of durability with a second discipline.
If you read between the lines, you can summize that you cannot use versatility to improve your durability.

Think about it though, a gamemaster who allows human adepts to use versatility to increase the durability of, for example, a spellcaster. You also have a warrior in the group. The caster learns durability from the warrior at circle 2, and never learns his own durability. Same LP cost, without ANY disadvantages.
Seems a bit overpowered, don't you think? I can't believe any GM would allow this.

I agree this appears overpowered. On the other hand with the fixed d6 karma for all races and the given racial modifiers, there's absolutely no reason to play a Human apart from Durability. So maybe this kind of weird usage would be allowed in the 3rd edition.

In ED3 revised its a second Core Discipline talent at Second Circle for all Disciplines and cannot be learned via Versatility.

As for the concept in question in ED3, I would allow him to learn Durability by Versatility, however regardless of what Discipline he learned it from, it would be 4/3, as the rating is still determined by his Discipline(s), and if he is a Second Circle spellcaster that is all he can get. If he were a multi-Disciplined Wizard/Warrior, I'd allow him to use the warrior Durability, but at a Journeyman point cost as per the second Discipline or wizard Durability at the Novice point cost for the First Discipline.

Do the talent options really undercut Human versatility that much?

Not in my opinion. Its an extra 15 talent slots that do not require a multi-Discipline path to learn. Versatility is excellent even when used as a tool to pick up more talent options, even those within your own talent options or another Discipline's core. And with only the core Discipline talents counting it means you can learn a lot more talents than you used to be able to in previous editions.