There are 2 angles to it
1. Personal
a. Less personal life, because you work more than 14-16 hours a day most of the times
b. You work on many things at a time, context switching and ensuring you don't lose out on understanding becomes trickier to manage.
c. Founder needs to trust you. This is a very serious role where you're trusted with a lot more information on founders professional as well as personal life. Without the trust, it becomes flaky.
2. Professionally
a. Knowing more about the business and curiosity to learn more
b. Problem solving at the same time
c. Get ready to pick all the boring tasks that founder/CEO doesn't like.
i. Includes everything boring as well as fun but have to make sure you're using your time efficiently.
ii. For all the small & manual tasks, it is better you hire an EA.
My transition plan has been of 6 months because we were in between massive hiring cycle in the US as well as India. So right now 60% to 70& of it was HR+ TA (More TA)