The description states attacks. If it does crit any spell, even then you're basically making a poor "investment" because you will probably eventually reach 100% crit chance even on Insane.
I can confirm that Stealth crits everything that can crit not just attacks.
In concrete terms Stealth helps out Barrier a lot and Time Shield not at all. This is perhaps not surprising since Anorthil have a crit/cunning focus. Anyway absolutely everything auto-crits that can crit in stealth. In the full scope of the game the Stealth becomes less powerful as you level since the crit chance goes from great to not as great compartively.
However it provides two advantages:
1) Its a very good crit mult addition, you would need roughly about three other talents from 3 different trees to get the same crit mult (for example combining the Thug and Shalore talents is about 50% +crit mult).
2) It allows you the budget other things into your equipment instead of crit chance.
One of the caveats to #2 is that often crit chance and crit mult go together on equipment, also if you need to budget stealth rating into equipment you get no advantage. So especially for melee who usually use equipment to augment stealth rating you may not see equipment returns. However you should in theory wind up with a straight up better crit mult than many builds and have more room in your loadout for other things since you need less Cunning and crit rate items. That may sound weird since Stealth rating is based on Cunning but many builds raise Cunning for the crit percent but it seems that by managing position and number of things in LOS you may get away with simply average Cunning.
Another caveat is that on the higher difficulties you tend to get more gear and better gear. There are WAY more rares and they drop pretty good stuff. You also gain levels faster.
So in higher diffculties leveraging an equipment advantage may also become less and less useful and the higher end crit rating will become available earlier thus doing a double whammy on the relative advantages.
All in all stealth is really not overpowered in anyway compared to other options, its just an out of the box effect basket you can get in other ways. However if you plan to run a strategy that will most likely keep you in stealth without a heavy equipment investment it is a very nice crit mult enhancer for only one skill tree, at the cost of a fairly heavy class point investment. From an adventurer point of view it may be a rather efficient way to get a lot of crit mult from a Cat point perspective but not a class point perspective (roughly same for class poiints at the cost of the mechancis of stealth itself).
I do not think stealth builds are really the height of min/maxing, I was making that adventurer to see how a more optimised version of a similar build worked out in Insane. I really don't think Stealth is appropiate in Madness (perhaps interesting to try just for grins). I haven't actually unlocked madness but the special effect of being "hunted" is most likely something that is very bad for stealth. There is another caveat there, in that Stealth seems to have an effect of making it much easier to lose mobs who want to chase you, perhaps that can help, but I suspect its nullified.
But I will say trying to make Stealth work out in Insane is kind of fun. Its an interesting extra wrinkle. It may become annoying instead of fun by the time you get to High peak.
I had heard Stealth is worhtless on Insane and I suspect that is probably true for melee, but I think it can work on Insane for ranged.
As for Bone Shield yeah its not working like I was thinking. I was assuming Bone shield was before Disruption Shield but its not. So its only real use together is for use as a back buffer.
In general Bone Shield itself is not a great overall mitigation tool. It is however an amazing alpha strike nullifier or buffer. I am currently running an arcane combat/brawler type build that I also am using the Bone/Disrupt shield I have had a bit more time to play with it and its not going to meet my original idea of using it as a frontloaded buffer to manager disrupt shield and mana since disrupt will intercept damage instead of BS when both are active. It may turn out to be useful though as a back buff for when Disrupt shield fails. A very large hit can still go through your disrupt shield and it seems possible that having Bone shield as a barrier of last resort may be useful. This scheme seems non-ideal in that when Disrupt fails you essentially get a DOT that will also diminish BS. However it may not matter really in that the idea is to simply put a hard stop on something extremely nasty so you can get outta dodge and/or give you some margin of error.
In the end though if I can get Disrupt shield to work the way I want then Bone shield was probably a waste and have the Unarmed Disciplined combo kick to remove 5 sustains of any type would be three points cheaper and very powerful on Insane+. Or possibly speed control as this build is meant to be fast. Or possibly Sun for path of the sun. Or another inscription. Or the tree with corrupted negation and spell crit sustain.
I will see how it works out and if BS offers a use in the idea of a sort ablative hard stop as a complement to true mitigation of disrupt shield + aegis (this build has temporal too) but I have a suspicion that I should have taken another tree. I never use Bone shield as a main mitigator, but I like it a lot for use as very nice margin of error thing. With perfect play and certain tools (track, clairvoyance) Bone Shield is of lesser value, but as an anti-WTF skill it has value when used in combination with a REAL defense.
Mex is probably right that in the end Unstoppable is the pure min/max way. But I don't like it really, I feel its kind of uninteresting. Its just binary effect set in stone effect. I just can't see myself playing a whole game using it. And bloodthrist would work well with the above build since it causes bleeding all the time (stone tree) and that would give very good resistance penatration.
Its true that Bloodthrist instead of Bone would almsot certainly make the build better. But I was kind of purposely trying to make it not use Unstoppable.
You can even do Unstoppable and Disruption together and I may try that on Madness if I ever get around to such a thing when I unlock it and if this build works out. But probably not I just don't like unstoppable.