To add to the debate about critical successes and fais being RAW: they are not. There is a part in the DMG where they talk about the DMs possibility to implement such a system, but even that paragraph emphazises that the RAW is exactly not doing that:
Rolling a 20 or a 1 on an ability check or a saving throw doesn't normally have any special effect. However, you can choose to take such an exceptional roll into account when adjudicating the outcome. It's up to you to determine how this manifests in the game. An easy approach is to increase the impact of the success or failure. For example, rolling a 1 on a failed attempt to pick a lock might break the thieves' tools being used, and rolling a 20 on a successful Intelligence (Investigation) check might reveal an extra clue.
This part of the DMG also does not have the "Optional rule" tagging. Things like flanking and facing have it. This means critical fails/successes are not an optional rule, they are merely mentioned under Rule 0 "The DM is the final arbiter what happens in the game".
