Can somebody please explain me why former members are considered unreliable by some scholars because I don't understand?

Is this your way of saying that you don't think NRM scholars who argue thus are acting in good faith? 

I have to admit though that the testimonies of a fresh ex may be colored by a sense of betrayal.

Yes, of course, Andries.  You keep making that point on behalf of the NRM scholars who you won't really discuss. But isn't this the same as any complainant about anything?  Any whistle-blower, any criminal complainant, any civil litigant ... anyone can exaggerate.  So what?  The question is, are they?  What's the evidence one way or the other?

The NRM scholars you keep quoting seem to use that potential as an excuse to dismiss or discount ex-followers' accounts out of hand.  That's ridiculous.  Could you imagine if that's how the courts worked?