Physical keys.
The guest arrives at 1am because their flight got delayed. The lockbox jammed. The backup copy is at your mom's place. You're out of town.
And your phone starts ringing. Once. Three times. Seven. Until you answer, get dressed, drive over, open the door, apologize, smile through the review they'll write the next day.
Every guest is another roll of the dice. Every physical key is a small lottery you'll eventually lose.