The original Impervious Hands might seem weak, since the defense was focused only on the hands. But if Lex used it, it would not really be an issue since the naturally defensive capability of his body was more than enough to suffer most attacks coming his way unharmed. In exchange, the gain his hands would get was absolutely phenomenal. In fact, his hands became even harder to hurt than when he would be using the normal version of his new technique Harden.
Considering the fact that the technique gave such great benefits without any serious drawbacks already made it amazing. Once it was upgraded, however, the technique entered a whole new level.
To start with, the cost of the technique went up manifold. His spirit energy running out had not been a concern for Lex for a long time, but this technique made it a real concern. At his current capacity, Lex could only use the technique three times before he'd be completely drained of all his energy.