What would be a good buff to the theme is multiplicative system instead of additive:
1 shard would give 50% chance of critical hit/evade
2+ shards would work after each other, i.e. 1st tested if it works or fails 50%, if failed, then the next one is tested and so on.
So as a result, the chance of failure (no effect) goes down geometrically 1/2, 1/4, 1/8, 1/16, 1/32, 1/64 for 1-to-6 shards in play respectively.
This way even with 6 shards the effect is still not guaranteed, but very likely (63/64 chance), on the other hand, even 1 shard gives a 50% chance, so it is not negligible.