http://log-horizon.wikia.com/wiki/Overskills
http://log-horizon.wikia.com/wiki/Isuzu
According to these pages, Isuzu's 43rd Song had an actual, magical effect -- shifting rubble around to protect people.
Is this just wiki-editor nonsense, or verified by the light novel?
I checked the Web version, and there is this part in the last chapter of 'Route 43':
波紋のように広がる詩は世界に承認を受けて産声を上げた。指先から卵ほどの大きさの小石が浮かび上がった。あらゆる場所でリスのように踊り出すそれらは即席の防御となって、戦場のすべての攻撃に体当たりをして、魔法をそらそうと努めた。
"The Song reached out like a ripple as it burst out a birth cry, the World accepting its existence. Pebbles from the size of the tip of her fingertips to the size of eggs floated up. As they started to dance around like squirrels everywhere they turned into a make-shift defense line, charging into every single attack on the battlefield, attempting to deflect any Magic."
So,
technically yes, though it is more "pebbles" than "rubble", in fact, the above portion is immediately followed with this.
本当にちっぽけな詩だった。所詮は小さな石が踊るに過ぎぬそれは効果も小さく、僅かな足しになるかどうかも怪しかったが、五十鈴は詩を諦めなかった。
"It was a really weak Song. All the little it would do was to make small rocks dance, and it was doubtful that it was even worth it, but Isuzu refused to stop singing."
It isn't a "powerful" spell; in fact is seems that it is the opposite of it. The point of the song from Isuzu's POV isn't the magical factor:
いま誰にも届かないかもしれないという絶望は、いつか誰かに届くかもしれないという祝福そのものだ。そんな理屈を必要とする自分は弱虫かもしれないと五十鈴は思ったけれど、でも希望ひとつで世界の果てまでだって歩いて行ける。それがロックだ。
"The despair of that she would reach nobody came hand in hand with the blessing that it would reach someone someday. Even requiring such a rationale made Isuzu think of herself as being weak, but even then with hope alone she could walk towards the ends of the World. That's Rock."
いま五十鈴は音楽/43なのだ。世界は五十鈴のものだった。
"Now, Isuzu was Music/43. The World was Isuzu herself."
誰にでもこの旋律が届けばいい。星屑のように砕け散りふりそそげ。無力に憤る〈大地人〉にも、故郷を求めて子どものように泣いている〈オデッセイア騎士団〉にも。
"She wished that this Melody would reach out to everybody. That it would fall upon all like glittering stardust; both upon the 'Landers' who fumed at their own weakness, and the 'Odyssea Knights' who cried for their home like children."
It is that she genuinely believes that her song is worth something to the people, and the situation, around her; I'm thinking that the magical factor is just a bonus. As Soujirou and such say, an Overskill isn't really much of a big deal in itself.