You are going to introduce bugs
Human concentration does not last for long time, and it decreases drastically if your brain doesn't rest properly.
We are already introducing bugs when we are extremely focused and well rested in the morning, so it's easy to imagine the disaster that can happen at 10PM.
8 hours of mental work a day is more than enough for your brain.
Your changes cannot be promptly reviewed
Stories team estimation goes wild
One of the most painful part of any agile team is the stories estimation (less in Kanban, more in Scrum). It is painful, stressful, long and it is tiring for all the participants. Estimations are based on stories complexity and time. The time is the 8 hours per 5 days a week. Now, all this estimation gets distorted and so useless when team members regularly work extra hours. Team will get frustrated.
You can create tension between team members
You create wrong expectations
You lose a great part of your life