This is not confirmed, but it's obvious that Imprisoned Felmaw's RNG isn't purely random. Its RNG is 50% face, 50% board. This means that adding more minions (than 1) on the board won't help you avoid an attack to the face.
Is Archdruid Naralex the same? Because I swear I get Nightmare like 50% of the time.
That's interesting, if you could link the source of the confirmation of this by Blizzard or an extensive study which shows this is how it works then that would be great.
Helpful tip for future - just list sources in your post. It clears up a lot of confusion or people rejecting what you're saying. Not listing them could appear like you don't have one and you're just making it up based on 'feelings'.