I don't think it's good for making maps on its own because some features it lacks, but it's good in accompanying another program. I use Affinity and Other World Mapper as part of my chain. I don't regret buying them, especially if you wait until Black Friday it will probably be half off.

Let's look at features. Affinity Photo can now put text on a vector curve which is more powerful than Photoshop, unless that's changed since I last used it, but it doesn't have the hard bezel that some people use to make topographic mountains. It also won't let you chain a bunch of strokes together on a shape, or use a texture fill, both of which are very useful for mapping. So, you'll find making an echo outline a pain in the butt compared to Photoshop or Clip Studio Paint, you can't even do an expand-stroke the selection-expand-stroke the selection chain because of how their 'outline' works... it makes a selection mask of what you outlined instead. So I think you might have to make a shape with a gradient outline instead for the easiest way.

Its batch processing is also different than Photoshop's. But it does have options that will work for things like say, having made a sheet of assets that you want to save separately, or running an action over an entire file folder of images.

I am not a fan of its custom brush handling, and have never made cartography brushes in Affinity... I use Clip Studio Paint for the drawing part of my flow most of the time. There are a few exceptions but it just doesn't feel like a good program for painting in, inking and coloring sure, but not painting. Affinity Design is good if you want to make decorations with vector.

The lack of a texture-to-layer fill does make it inconvenient for battlemap creation. I believe it has a form of a texture fill, but it isn't the same as making a saved texture fill / layer style in Photoshop... you have to open up a file each time to fill in that file as a bitmap fill.

Anyway, it has its place. I'm glad I bought them. I use them often, just not for 100% of map making.