Trouble is, when evident mind-killing and breakdown of polite discourse occurs, it is the less bad failure mode. In that case, it is at least clear that something went wrong. The really bad failure mode is when the discussion resembles a rational discourse, but is actually a horrible happy death spiral. In such situations, the conclusions may seem rational and informative, but are in fact awfully remote from reality, or at best right in a stopped-clock sort of way -- and the lack of discourse breakdown is interpreted as a successful exercise in rationality, whereas in fact it's merely because nobody stepped in to spoil the fun by trying to draw it closer to reality. (The latter, of course, is likely to cause mind-killing and discourse breakdown, thus making the messenger look like the guilty party.)

Hence the crystal healing analogy I made in another comment, which may sound extreme but is in fact, in my opinion, quite pertinent. In both cases, a volatile mix of biases, preconceptions, wishful thinking, etc. produces entirely spurious conclusions about how the world works and how to deal with it, which are then happily accepted in a self-congratulatory way, even though the process by which they were arrived at couldn't stand up to any intellectual scrutiny.

I've seen some such discussions get quite bad but I've seen others where apparently calm rational discussion took place.

That's my experience, too. I have seen progress being made in some of the discussions about gender, even though they can be frustrating. But perhaps I'm focusing on the exchanges that I was involved in.