I don't know for sure that a caplock is less an eye hazard than a flinter. A flinter produces small but very sharp pieces of flint, but the gas jet from the touchhole might tend to vent it outboard. A caplock can produce larger pieces of copper cap fragments, but the nipple angles back toward your eye. Here's a picture of a caplock going off that I caught at just the right millisecond:
And here's another taken a couple of years later, also percussion, but a different gun:
The moral to the story, I guess, is not that one ignition system is riskier than the other, but that you should always wear dependable eye protection.