Since most social anarchists hold that resources should be allocated according to 'need', decisions would need to be made to determine who in the community has 'need' of the biggest shares of resources.
I know ancoms claim every individual will just take whatever they 'need' (want) from communal stores, but I'm going to ignore that because it's really not practical in an industrial society - resources aren't infinite and no one is going to spend their life doing gruelling manual labor and then just give everything they produce away to some random asshole who shows up at the communal store with a dumpster truck and says "I need the community's entire monthly output today, load it up". For some reason ancoms think assholes would cease to exist in a communist society.
In a real world implementation of industrial communism, communities will quickly impose limits on what can be taken from communal stores after a few people take way more than they have any right to and other people go hungry as a result, even though they labor 8 hours a day.
So for resources to be allocated according to need, you'll need some kind of deciding body in place to judge what each person's needs are; what resources each person should be given.
Giving that body this power will mean certain favored groups / individuals will be rewarded and less desirable groups / individuals will be neglected, or even punished. It's always felt like a recipe for bureaucracy, corruption and exploitation to me.
The only social anarchist tendency that made a modicum of sense in my mind was anarcho-collectivism, because at least the workers would receive the direct value of their labor hours instead of having external bodies decide how much value / worth to assign to them as a person.
If you're gonna spend your life toiling in a factory or farm to produce goods for other people, would you really want a bureaucrat or a commitee or even a direct voter body deciding how much you deserve for that labor, while giving someone who does the same job more than you because of potentially biased reasons?