You mean theoretical and uncertain like the experiment they're doing on a global scale now? When did we ever have 130 nations all agree on the same corporate tax rate?

Not that I think it's necessarily bad, it's probably better than the current system which is very broken. But can you explain how and why that is actually a better idea than introducing a BAT system? Again, both are theoretical and have never been tried, so you should have very good research to back up a claim that the global flat rate tax is the better idea.