It should be fine, but I'm against any ability that relies on criticals to activate. Just feels like garbage to have a big feature only apply 5% of the time.

Personally, I'd just change it to "When you have less than half of your life, you may treat any natural 1 on an attack die as a natural 20". Ties in with the Barbarian-Smash concept, while adding synergy from your Rage dice, while making criticals more relevant.

In order to get the most value out of it, a Barbarian would have to Reckless Attack when they had less than 50% of their health, which means they're also taking more damage. High Risk = High Reward.