I find the shield skill debuff component of shatter very underwhelming. I don't really feel it. I only feel the initial no-shield effect that lasts a few seconds at high mana and success. I think it's to do with combat in general and not the shield skill debuff. Shield skill is only half, usually less than half, of the defense contest equation, it seems, so it can only really be half as effective (or less) as a debuff to another skill. Not sure if this explains my experience but it makes intuitive sense. Could shatter maybe do something else, too, like debuff armor or increase armor stealth hindrance for the duration?