Th biggest problem I have found each time I have evaluated gimp, is how long it takes to get work done.

In Photoshop so much of the workflow can be done with non destructive adjustment layers and smart object. So I can import a raw file, apply contrast and B&W filters perhaps a gradient, and then go back and make a change to the raw object. Because its all adjustment layers you don't have to redo any of that work.

In gimp you import your file, make an adjustment and if you want to change the raw file, well oops, you have to recreate every layer again.

If the gimp team implements this it will be a significant win for users.