Hidden may be fine for Skirmish, I'm not sure, I haven't used it much there. For Campaign however I've found it too weak. I've played 4 campaigns now where hide has been used every mission and I've found the -2 accuracy has never been a factor.

The game is made to work with characters getting to point blank for melee, with fast speeds and small tiles. The -2 accuracy on hidden is too trivial to overcome. This is particularly true in campaign over skirmish with strain for movement, +accuracy weapon mods, character abilities and near objective deployment points all make it easy to get to ranges where the -2 will have no effect.

The one place I have found hide to be effective has been with the Stealth class card. This coupled with hidden's -2 accuracy completely neuters Biv's main ability which several of his class abilities hinge on. It also turns a lot of melee weapons in the deck into dead draws. While this is actually a problem with the broken Stealth card, which unthematically punishes melee far more than ranged, and should itself be fixed, it is possible to fix it just fixing hidden.

So my current thought is:

Hidden:

While attacking: + surge, must discard

While defending you may discard for + evade and if the attack is RNG then also -2 accuracy.

By making hidden go from completely useless for defense to often useful once, it makes this much closer to the other conditions for how powerful it is. I also think completely removing the -2 accuracy would be perfectly fine, as I said, it's never come up as being useful, but I left it on to make people happy.