I'd say "in evey case". And I'd say this happens whether or not there's an explicit hierarchy. Being articulate, or having knowledge or contacts, will get you more attention. The only way to mitigate this is through explicit processes, which have to be regularly reviewed. Meetings have to be chaired strictly. Everybody has to sign-up to the agreed processes.

These processes are time-consuming, and decisions can be slow. It becomes worse the bigger the organisation. I don't know how non-hierarchical decision-making can work in an enterprise where there are significant assets at stake; I've only ever seen it work in voluntary political associations in which non-hierarchical organisation is one of the prime objectives.