How hard can it be? (my favorite phrase, in addition to “It depends”)
Isn’t it just about opening Windows Explorer and dragging over the files where they should be? Or can you not only use a script or a tool to copy all files to SharePoint and then start cleaning? No, it’s not that simple!
But it can be simple if you have planned the work and know what to do (otherwise I guarantee that you drive in the ditch already at the start).
“It is important to move the right information to the right place, the right way and in the right time.”
When we talk about migration, it is also important to distinguish between moving and copying information. When migrating, it is, almost exclusively, about moving files (since one does not want a file to exist in more than one place).
Briefly summarized: “What shall be moved, where shall it be moved to, how shall you move it, when shall you move it and what shall you do when it is moved?”
Robert Crane has developed a very good and simple framework that gives you tips on what you should think about and in what order you should do the job.
Here you can read more about it: “A framework for file migrations to Microsoft 365“.
The tools
What tools can you use for a migration? Here are links to two very valuable (basically indispensable) tools that I use when I help different organizations and companies to migrate files to Office 365.
What should you use when? The answer is also here “It depends”) 
Providing Tips 