Not sure why you think that. I've been a member and (occasional) moderator on a number of closed forums. All of them have had spam posts to varying degrees.
Yes there is. Flamewars drive away the majority of users who can't be bothered wading through all the bile and don't want to find themselves a target of it.
If flamewars are left to run wild, before long the only people still participating will be the trolls.
It doesn't have to be as hard as you're making it sound.
Criticism should be directed at ideas/opinions and disputable 'facts', not at people. Mostly, it is pretty clear when that line is being crossed.
Any forum with a membership of several thousand or more is likely to occasionally see threads veering off topic and getting mired into the weeds, including political and/or religious views. It happened periodically at DPR, so there's no reason to assume it won't happen here too.
Realistically, whether they're called moderators or not, having people who have admin privileges and run the site is unavoidable.
Ideally, their actions should be transparent, rules should be clear and applied consistently and there should be as little intervention as possible. But I don't think we should kid ourselves about the need for some guardrails.