I don't know anything about FX sails, but I've done business with Ullman for years (decades?) and I'm a big fan.
IMO, the *best* lofts are the local-and-locally-owned ones. Ballard Sails in Seattle, for example, has a great reputation for quality and service. When you call, you're talking to the people who will actually be building your sails, not just taking an order and unpacking a box from offshore a few weeks later.
At the other end of the spectrum... some of the big name-brand ('fast rags come in blue bags") seem to be uninterested in small and one-off orders. If you're buying a whole $uit of sails for a high-end racing program, you'll get their attention. On the other hand, if you're buying a single sail and it isn't a "big order", in my experience it can be hard to get them to return your calls.
Ullman is at a sweet-spot in the middle. The lofts are locally-owned and locally-run, while part of a large network that includes top-level sail design, cloth choices and such. My most recent (2017) purchase was an A2 spinnaker. After getting quotes from UK, North and a few others, I went with Ullman... primarily because they provide a great experience (walking through the choices of sailcloth, size, shape, etc), through measuring the actual boat and installing the sail when done, and have built a great reputation for service even after the check has cleared.
All things being equal, any competent loft (local or online) can buy some cloth and build a decent sail. What you're paying for, in a good local-loft deal, is a "relationship" - you're paying for people who will come to the phone, pay attention to your concerns, and take care of the details. To me.... that's worth the extra cost.
Bruce