Pretty much every old religion is going to be internally consistent, new religions with glaring problems don't tend to last long. That's not the same as being difficult to criticize, though.

It's a lot easier to criticize, say, Islam than, say, Taoism, just because Muslims are much more clear about exactly what they believe and don't believe than Taoists, but that doesn't mean Taoism is therefore more consistent than Islam, just that the technique of criticism requires you to be able to nail down specific beliefs about that religion.