the problem is that there are a lot that are not, and many have explicitly stated that they want to disrupt other instances
It might seem that way but I don't actually think this extends much beyond young posters getting hyped up about how leftist they are, in the same way a 17 year old newly converted atheist is going to be mildly obnoxious to interact with in any online space.
explicitly stated that they will not moderate their users for things they do on other instances
See I saw that but I actually read it the other way; there's a lot of dumb ways to run afoul of moderation on hexbear, not liking vegans, complaining about certain users, just relatively easy to catch a ban for totally asinine stuff. So I took that to mean that they weren't going to hold everyone to account for hexbears code of conduct outside of hexbear spaces, so I wouldn't catch a ban over there for talking about the logical incoherence of certain arguments for veganism here.
I can't imagine they mean to say they're willing to serve as a home base for disruptive and rule-breaking behavior on other instances, and they should absolutely be willing to impose bans for that (at the threat of defederating if not), and if they are saying that they should clearly reconsider, but I still think we should give these systems the opportunity to actually develop as opposed to preemptively siloing us all. lemmy.world admins should open up a dialogue with hexbear ones to see if there is some path forward if they actually do have considers that aren't just ideological disagreements.
The hexbear userbase has a tendency to attach outsize importance to their posting activities, when in reality it's just an anodyne shitposting website with general leftist politics. When a admin makes an argument to the effect "leftism is better served by you playing nicely on other instances", they're playing to that tendency to try to get everyone to behave, not because hexbear can actually meaningfully engage in coordinated ideological warfare campaigns; we can't even agree with each other.
There's an old propaganda poster about carpooling during the war that works on the same conceit.
In reality, the only thing you're at risk of by federating with hexbear is the occasional post "look at what this liberal is saying on libby.world", and if that triggers a brigade, it's clearly visible due to federating tracking home instances, and can be dealt with (unlike brigading on reddit). This can (and does) even happen without federating, but you lose the biggest tool in your arsenal to actually see it while unfederated.