Yes but that also works on physical weapons it doesn't specify stuff like that, and it just ends up making the other enemy servants weaker with every other engagement lancer is present in. And since we're the attackers we can possibly abuse that. Send in Lancelot, Archer covers him, Lancer uses his skill on enemy Lancer his magical ancient lance suddenly becomes less effective, sorry Diarmuid your already unflashy weapon now has more difficult time cutting through armor. Also Caster your book of deep, and bizarre magic can now only conjure two monsters instead of six. 

Its effects are too potent, and too general to be allowed in my eyes.