Unlike GIMP it doesn't work on GNU/Linux.

Affinity Photo / Designer are very very far from Photoshop. Affinity Photo is a replacement for Lightroom (raw photo editors), and Affinity Designer is a replacement for Adobe Illustrator (vector graphics).

Although you can do vectors and photo editing in Photoshop, the main purpose of PS is raster painting, which neither Affinity Photo nor Designer are remotely good at.

As such, neither of them could replace GIMP or Photoshop.

Affinity Photo is very different from Lightroom and it is very good for general purpose raster editing including illustration.

I've used GIMP a lot and finally switched to Affinity Photo+Designer. It is, I must admit, much better for almost everything, the only thing GIMP still handles the best is selection and cropping.

Affinity Photo even supports Photoshop plugins (although I have not tried that).

Affinity Designer fits somewhere between PS and AI.

I find it better than both for doing web design and game graphics.

Affinity photo is more lightroom + photoshop mix

Affinity designer is illustrator first + photoshop pixel support

I prefer using AffinityDesigner and AffinityPhoto, fast development, loads quicker, the export tool is way better than adobe products, just all around easier to use.