We are using this product for about two years now in our organization (B2C) where we use it as part of a customer journey management solution that has been integrated into Salesforce Marketing Cloud.
The biggest benefit so far was how easy it made the workflows definition process - very simple! And it's also great seeing those flows implemented by other users/organizations who have different business requirements than us but still can leverage them thanks to their "universal" API support. No real downsides at this point, maybe just some minor bugs here or there when implementing new features / changes from time to time? But overall I think people should give it try first before considering any alternatives out there. For what concerns user experience/user interface customization options, since these products tend to be proprietary solutions you might end up stuck if your needs change over time without being able to customize things yourself due.