What are the technical challenges while migrating to Sharepoint 2013 from Sharepoint 2010/2007

If you have custom controls then you need to re-write code for that.

You have to be experte in migration and it goes like

Sharepoint2007 >> Sharepoint2010 and then sharepoint2013 - not directly to sharepoint2013.

ideally one more temp server for sharepoint2010 and then test server sharepoint2013

Hi, The technical challenges are based on the type of migration that you have.
If you have OOB site no custom coding then minimum technical challenges will be there.
If you have SharePoint2007/10 with custom solutions then you also need to upgrade the solutions to SharePoint2013 version manually and deploy separately as they are not part of your content DBs of specific web applications.
you can follow the hybrid or in place upgrade methods as per the type of your older SharePoint environment.
you can get too much material for upgrade from Microsoft site very easily.
I hope that will help.
With best regards, Ibrahim