Let me qualify my remarks with- I am not an engineer but I am a designer so that being said. I think the issue you are having is one of physics the bucket has a curved surface, the threads on the nipples lie in a flat plane so when water is in the bucket, you have two differing forces at play.
What has worked for us is to use cat litter buckets (they are about square with flat sides) we use PVC fittings (1/2 male thread to slip with the male threads going through the bucket) and put a water hose rubber washer on the inside between the above fitting and a female threaded fitting to sandwich the bucket between the fittings- on the outside, we make a cross pipe that we then drill and tap to accept the horizontal or vertical nipples.
We have used this for the past 5 yrs and no issues so far
I've got nipples in a curved sided bucket and they've never leaked nor has the bucket split. It's been in use since last summer and even froze solid a couple times this winter before I got around to putting a heater on it.
Two thoughts come to mind. First, maybe the bucket is too brittle, or maybe you drilled the hole too small. Or perhaps a combination of the two.