The advantage of an unincorporated association is that it is really easy to set up and there are no statutory rules about governance, formality, reporting, or anything like that. The difficulty with an unincorporated association is that it has no legal personality of its own and so the association as such cannot own property or make contracts. Association assets (which is anything from a club wheel barrow to the site lease) have to be owned in trust by association members and while that's entirely possible trust law is a bit complicated and formal and sometimes trustees forget what their responsibilities are. Committee members are personally liable for the association's debts and liabilities, and the liability is joint and several, so basically any one committee member can find themselves entirely responsible for a thingy-up that they had nothing to do with, so if you're managing a site as an unincorporated association the committee really need to make sure that they are managing their financial affairs prudently and that health and safety is managed well and liability for 3rd-party claims is insured against.
Incorporation creates a legal personality all of its own, so in its own right the association can own property, make contracts, sue and be sued. Generally speaking liability is limited so there is much less personal risk for the directors. The downside is that there is more formality - there needs to be a written constitution which is registered with one of the registrars, and accounts and reports need to be filed, and there are penalties for missing deadlines. There are a number of different forms of incorporation and it's not necessarily easy to work out which is most appropriate and the choice also bears on whether or not you can or want to register as a charity. That said there are plenty of incorporated allotment associations so once you're up and running it's not too bad as long as the committee takes its duties seriously.
