Member Mojo - multiple Associations under one subscription? it may well be scalable, but that's not quite the right question. The question you should ask is whether it can be partitioned, so that different societies can manage their own membership separately. You could ask the same question of a large society (like ODG, which has 15 branches).
The answer is no. The consequence is that whoever has access to manage the data would have access to it all.
LoveAdmin can be segmented to handle structured organisations, and is used by at least one ringing society, but it has a different charging model.
ODG is in fact intending to use MemberMojo, and I am sure it will be made to work, but I doubt it will prove as useful as a system that could support separately managed parts of an organisation.
As for different societies using it, that would be a step too far.