Martials don't even get anything from a 5th level multiclass into another martial class

If you roll a Nat 20 or Nat 1 it raises or lowers the outcome from a crit success, success, fail, or crit fail. So Nat 20 on a roll that's still ten below the DC takes it from a crit fail to a fail. It stops a Nat 20 on an impossible task from being a success if your skill just isn't good enough in any way. We like playing it this way cause if our bonuses are so good (reflecting high training and skill) we can auto pass certain low level checks even on a Nat 1. It still means a Nat 1 is somewhat impactful as it stops an auto crit

[Edit]: Adding an example Rogue attempts to pick a 'complex' lock with a low DC. Complex locks require multiple successes to actually unlock, and a crit success on a pick lock check counts as 2 successes towards opening it. Rogue has +22 to picking locks, lock DC is 10. With a roll of 1, the result is 23, which is more than 10 above the DC meaning critical success. But since its a Nat 1 it drops to success. So 5% chance of only getting 1 success towards the multiple required to open the lock. Picking a lock requires an action in combat to do this, so can add a bit more intensity if the party desperately needs to open the lock quickly. If it was outside of combat, DM would just say we unlocked it since its impossible to fail meaningfully if given enough time.

On the other end, if the a different party members bonus is +4 because they are untrained and the DC is 35, a nat 20 gives a 24 as a result which is still 10 below a success, so a crit fail. Since it was a nat 20, the result goes up from a crit fail, to just a fail meaning it mitigates the worst part of the result. And FYI, a crit fail on picking a lock breaks your lockpicks so there's extra outcomes and narrative results gained by using the crit fail, fail, success, and crit success rules