I'm wary of any "religious law" even though I know that in Islamic countries, religious jurisprudence often makes up a huge part of (and influences the rest of) secular law.

My western, liberal democratic mind is wary of any time someone says "God told me ... and therefore you must do ..." At least in secular law, you can appeal.

As you say, I suppose it can be done well. But the examples that come immediately to mind are places where it has not gone well at all.