Matt,
I am reading this only now. We are renting space for one of our sites in Seville (Spain), and it came with UniFi WAPs. I have very little experience with them. But I can tell you that we're getting way more complaints than I would expect, at least if compared with similar setting (in terms of nr. of people, nr. of waps, and size of the areas to be covered) where we use Cisco Aironet WAPs. I can tell you that in my experience Aironet WAPs are quite stable and effective. Have you ruled out Cisco for some reason? In any case, I have NO actual data to think UniFi is "bad" - we still need to sort out what is happening in our Seville offices -, but, at present, I am not too confident on their quality.

As to a controller, Aironets are so stable that we basically never have to reconfigure them. We have 40 devices in the building where I work, and we have no controller. I'd definitely benefit from one, but not hugely.
I agree that a controller will bring benefits to roaming, in theory, as mentioned above. BUT, as a matter of fact, in the 40 WAPs setup we have, roaming never seemed to be an issue. So, I would assume that Aironets will work well in smaller environments.

Having said that, I do agree that peopagation of WiFi signal is unpredictable. I recommend a WiFi survey to optimize WAP location. Without it, we wouldn't have made it - I too work in a 2 centuries old building with think concrete walls, metal pipes long forgot in them, and what not.