How to switch CRM’s???

What are the biggest issues and problems in switching CRM's?

I am looking at other services. I have not nailed down which service I will go with yet.

For context I am in the home services space. I pressure wash, clean windows and sometimes I will take on solar panel cleaning jobs.

I'm not looking for CRM suggestions just advice on switching. I know I will have to worry about data migration and training my team on the new software. What will be my biggest challenges? On average how long does it take to migrate data?