I went and looked at the website. I do think it’s interesting to have someone try to reinvent that saddle as it really is a very 18th century technology we are using!
My impression is though that for all the modularity, there aren’t really much options for addressing how the tree fits the horse. Besides gullet width and height, the crucial thing in saddle fit is that the tree has to be the correct curve for the horse’s back. The ideal is getting a saddle built from scratch after fitting a bare tree to the back of the horse (around here only handicraft Western saddle makers do this). Or with an English saddle, choosing the best match you can find and having the wool panels flocked to fit.
Looking at Bua, I think that if it was a good fit for your horse, you’d be fine. And if it wasn’t a good fit for your horse, it would be unusable. It might be interesting if they provided a range of trees, and perhaps a range of panels. As it is, it’s modular (with all the potential structural issues that entails) but not particularly customizable.
I also don’t love the idea of having my saddle rely on velcro and zippers. I know what happens to velcro and zippers at the barn!