I’ve made a few surfboard fins on my fairly stock Shapeoko 2, so I suspect it wouldn’t be a problem with an x-carve. Fins for a longboard are fairly large, and although I made a few, one I designed was too big to be made on the Shapeoko (I’m about to upgrade my machine, and then it will be able to make larger fins). Most of my fins were made of wood, but some of layered fiberglass, and the first ones were plywood. I used Fusion360 to design the fins, and for center fins had a process whereby I’d flip the fin over to do the other side. I made several passes and went slowly, and the final result was very good. It took a very long time to carve each fin (but with my upgrade it should go faster), but the results were great.
I spent a lot of time thinking about how I wanted to do the halo, and the process I ended up with is a bit different from the way most people do it. It is more complicated, but I think the results were worth it. You can read about it here:
That page also has several pictures of the fin and my Shapeoko, as well as a picture of the finished fin.
Once I made some fins out of layered fiberglass, and you can read about it here:
That page also has a photo of the finished fins. It was MUCH easier and quicker to make the fins out of fiberglass, but I didn’t like making the layered fiberglass panels.
I am really bad at CAD and the hardest part of the whole process for me was figuring out how to do what I wanted to do in Fusion360 (mostly because of my halo process). But it was a very good learning experience.
Making surfboard fins on a CNC is a lot of fun, and I made some for friends. But I certainly wouldn’t do it as a money-making venture.