1. They burn out and clock out mentally or leave altogether
2. They develop psychological issues and destroy their personal life, leading to an unbalanced and unsustainable situation
3. They expect something from the org in return, which normally the org can’t/won’t match (either in the way of product ownership or compensation or role).
Someone like that needs to be constrained. Keep them on the edge and eager to contribute but don’t let them free reign. If the situation and culture allow, try help out to fill the void in their personal life (I ordered one of our guys to leave early twice a week to spend time with his girlfriend because I knew his dedication to work is destroying his relationship. I sent couple of others to weekends in hotels with their SO after long sprints).
It’s a bit odd you’re setting your employees up to maintain their sex lives but I’m sure they appreciate the forced time off after stressful times