The Yin Yang Sanctuary was like a moving Sacred Relic Field, but obviously with only one of its advantages. The real Sacred Relic keeps the Godly Pressure out, and it also weakens the Godly Creatures. So, not only does it prevent the cultivators from having their strengths weakened, but it also weakens the Godly Creatures. That's another reason the Godly Creatures attack in hordes: to overcome the Godly Pressure nullification with their sheer numbers, each emitting Godly Pressure.
The Sanctuary, on the other hand, only prevents those inside from being weakened by Godly Pressure. It doesn't weaken the Godly Creatures. So, yes, it was only half as good. Because of that, the twins' group was debating whether they should really buy it or not. It was still very, very good, but points were obviously hard to come by.
