I had an interesting situation arise yesterday. I was out with a buddy who I am teaching to reload to test fire his new rounds. On the first round the primer looked a bit flattened and had worked out a bit. These are once fired brass, head stamps looked good, no ejector marks. The whole situation was odd, and I knew we couldn't be over pressure because the Chronograph. Chronograph was saying we were perfect for a starting load.
My point is that primers are one indicator of high pressure and alone are not necessarily indicative of danger. Some loads just do funny things to primers and some primers are very susceptible to pressure. Primer reading is either hit or miss. I'd rather trust my Chrony.