Considering they make a lot, and I am talking 5000+ just arrived here before another four digit delivery earlier this year from what I heard, for Australia alone, I don't think I could even guess but would they have been making more than a million sets of wheels a year at peak before covid issues?
I would think the ratio of bad wheels is so very low compared to all the wheels with no issues out there being skated. That is not including flat spots from user error or other non manufacturing issues (like being run over or dragged by a vehicle) but I think I have had two sets over the years that had warranty issues, in maybe a few hundred in total, probably more.
The older Spitfire wheels did flatspot a lot, but the Formula Four seem to hold up a whole lot better. Sure some still get flatspots (any and every wheel does if they stick long enough on one point), but they are way ahead of what they were before 2013.
Combine that with an excellent warranty and service department, I think they have it pretty well covered, but there are always going to be some issues along the way, maybe more so with what is currently going on.
There is going to be another shortage soon enough, with supply of raw materials now causing production issues too, so that will be interesting to see play out.