Every so often the subject of clay ammo pops up. A lot of time it comes from new shooters. They usually ask about achieving better accuracy and state that they are using clay ammo. I, as well as many others on the forum, go on to tell them that clay has inconsistencies in size and weight that prevents them from being a truly viable option for best accuracy. Especially if you are pushing them with bands that are too heavy for them. The picture shows a good example of this. The ball on the left measures 12.37 mm and the one on the right is 13.89 mm. Both from the same batch. The difference is more apparent when in hand compared to the picture.These clay balls have very good consistency as far as actual roundness, but there is clearly a big difference in size and weight. Many are close enough that it wouldn't really matter, but there's no way I'm sorting through 2000 balls to separate into individual groups.
I do think that clay is a valuable training tool to new shooters. A fork hit with clay is much better than with steel. They are capable of doing a lot of damage should a shot go astray, but I'd still prefer a wayward shot to be clay and not steel or a marble. Once they are confident in their ability to at least keep shots inside a catchbox, I think it's time to move on to a more precise form of ammo. I still always use clay when I get a new frame, or make a more drastic change in my setup, as a little "just in case" insurance. Usually not more than 2 or 3 shots though.
On the flip side of that... If your style of shooting is just very casual walks in the woods shooting at various targets of opportunity, or backyard plinking, clay is capable of maintaining "minute of can" accuracy at reasonable distances.