Critical misses and failures mean that the fighters and other combat focused characters suffer the most, turning them into the least consistent combatants.
Why? A "critical miss" roll of 1 comes up a flat 5% of the time. Fighters make the most attacks. They will also then make the most critical failures. So the people who are best at fighting are those that critically fail at it most often.
In nearly every edition of Dungeons and Dragons, the strength of fighters comes from their combat ability. Turning them into the least effective combatant quickly neutralizes their main trait and contributions.
This is just a basic design consideration many Dungeon Masters may not consider. I come not only bearing knowledge of a problem, but also a solution.
Any class that relies on fighting, for example, any class that gets an 'extra attack' does not fumble on a 1, but instead must roll again, fumbling only if the second roll comes up 1. This causes fighters to critically miss .25% of the time and other classes to critically miss 5% of the time.
No muss, no fuss, and you're not twisting the balance of power towards spellcasters.