In general, I'm glad of bounded accuracy making the need for automatic (and critical) success/failures unnecessary. The problem for criticals is that it will on a d20 will happen 10% of the time (1 and 20), and this makes them fairly common, rather than special. One thing that was interesting about the critical hits of 3E was the confirmation check, making them much more rare than they are in 5E, and thus special. A critical failure should conversely work the same, keeping them both memorable and special.
I will agree that I feel that there should be an opposite to rolls. If there's a critical hit in 5E, then there should either be a critical miss or a minimum hit. A critical miss would be fairly easy: you grant advantage to the next attack roll from that target against you (you've opened yourself up to them in your attack) before the end of their next turn. In the rare event both have multiple attacks, and one has multiple critical misses, this would stack on the number of attacks. A minimal attack is easy to figure (hits exactly the AC), but harder to resolve. I'd probably go with half the damage dice with no minimum, but this makes it pretty weak for most weapons since you'd only get your modifier. Another option would be to set all dice to a 1, which feels really bad too.
I will agree that I feel that there should be an opposite to rolls. If there's a critical hit in 5E, then there should either be a critical miss or a minimum hit. A critical miss would be fairly easy: you grant advantage to the next attack roll from that target against you (you've opened yourself up to them in your attack) before the end of their next turn. In the rare event both have multiple attacks, and one has multiple critical misses, this would stack on the number of attacks. A minimal attack is easy to figure (hits exactly the AC), but harder to resolve. I'd probably go with half the damage dice with no minimum, but this makes it pretty weak for most weapons since you'd only get your modifier. Another option would be to set all dice to a 1, which feels really bad too.