The first trouble is that you are opening the doors to all sorts of extremists. Whether they're far left, far right or simply personally unhinged, they are as predatory as spammers. They have no qualms about abusing your good will and s**ing in the punch bowl.
Same thing happened with Gab and Minds, but also smaller bulletin boards I've seen over the years. You set up a free speech site, and next thing you have a bunch of Nazis festooning it with swastikas. They're just like spammers, except they're spamming ideology instead of work from home scams.
The second trouble is that Parler is a competitor to established media corporations, who have a strong incentive to shut them down. That's a reality that every new business needs to face: established firms do not want competition.
All the people running amok provide the established firms with plenty of ammo to tar the upstart's reputation. And while they are biased by their incentives, they feel they are acting justly because they see all the extremist lunatics there. AWS didn't fabricate those incidents on Parler, all those were real posts.