Ubiquiti has the Unifi line and the UISP line.

The Unifi line seems like it's geared for residential and small-medium businesses.

The UISP line, as the name implies, seems to be designed for ISPs.

Other than the the obvious differences inherent in each respective product, what's the difference between these?

Do the UISP devices have a controller like the Unifi controller?

Does the UISP offer certain features the Unifi doesn't?

I agree with your train of thought.

From my experience and opinion,
UISP is more for the provider (Fixed Wireless or even Fiber of some flavor)
Unifi is more for the small business/Enterprise network.
They can have some over laps, but really are designed to serve two different markets.

I'd say UISP has some additional features, but not sure you'd need or want them, unless in a ""Provider"" type market.
Just a few I can think of of the top of my head are Mapping of where you have fiber or wireless coverage of a given area.
Provisioning speed packages for individual customers,
Billing, invoicing and Credit Card Processing of each customer.
All the account data, contact info and notes on their setups,
etc...

Examples of some of our usage of what, where:
We use tons of UBNT radios (900 mhz, 2.4Ghz, 5Ghz, 24 Ghz, 11 Ghz and 24Ghz) on towers as Back Hauls, and APs....  
We also use a ton of UBNT radios (in 900 mhz, 2.4Ghz, 5Ghz) on customer's house/buildings as CPE type devices or HOPS to the next site.
We do most everything, set up, upgrading code, provisioning, trouble shooting, back ups, Restores, etc, etc,  manually, by logging in to the web/dashboard or GUI in each device and do what needs done.
Takes longer, but I don't trust the UISP server to not brick a tower full of customers or the tower's radios themselves.  
I look at a customer radio to make sure its not moving more than a few k of data before I upgrade code.  If a customer would pull power and reboot it in the middle of a firmware upgrade, it can and has bricked them.

I've seen versions of code that fixed a bug, but eliminated a Freq from the allowed list.  
If everyone on tower/AP Johnson Mountain was on say 5545 and that was the freq the new firmware eliminated, when the server batched them, they'd all reboot, load the new firmware and not connect to anything as they could not use that freq.  
Seen it happen, more than once.

Unifi.  I don't use it much, but my boss, the owner uses it and its gear to set up large Wifi setups, Cam systems, VOIP systems and enterprise switches and routers for hospitals, churches, hotels, RV sites, and other businesses.  

Hope that helps a bit.

Unifi and uisp are more like management platforms.  Unifi gear is managed with a unifi controller. Uisp gear is managed locally or with a uisp controller. It would be perfectly normal to use airmax or other uisp gear in an enterprise network for things like outdoor wireless bridges between buildings, but those devices don't need and won't work with a unifi controller, you would just manage them via their built in webui if you aren't an isp or have other need for a uisp controller.