Well, blocking SoW damage is clearly not a good design.
And player playing spells are also "attack". Sundials stops "creature attacking".
Why? Not saying I can't be convinced, but it doesn't seem hugely unconvetional to me. Spells are not "attacks" maybe in the english sense of the word, but pretty much every TCG ever makes a distinction between spells that deal damage and creatures attacking, even if they have creatures that deal their damage as spell damage. It allows for multiple layers of reduction or prevention as well as variety. Sundial already allows for affecting creatures that are otherwise unaffected (spell damage, momentum, freedom) so this is not completely without precedent.
It can accomplish the task quite well as it is.
This probably should not decrease Psion's damage unless you want to decrease ALL formes of damage. Otherwise, this currently reads as a standard DR shield that has benefits against Immaterial, which is pretty good already.
The problem is, especially now that SoW has been changed, is that many decks include it. Thus, if this shield does not affect wisdomed creatures, it is ineffectual against aether decks, and may in fact help other decks that run SoW for the non spell damage attack boost.