Simple RAW for 5e V: The Disadvantage Strikes Back

[E’Tallitnics]

[B]Q43

Can the -d4 from Bane negate a critical hit?

A43: No. A 20 on an attack roll is an automatic critical hit, no other modifiers are applied after that.

"If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC."

There are mechanics that can disallow a crit (adamantine armor, etc.).

WARNING! Per the RAW a "crit" is only on an Attack Roll. It does not apply to a Skill Check nor a Saving Throw. For those you still apply modifiers and need to meet or exceed the DC set by the DM.