I downloaded the trial and did some quick processing of some SHO data.
My usual workflow is to stack in APP, stretch/blend in PI and then use the color tools in PS to "season to taste".
This time I just pulled the stacked linear images into Affinity Photo (AP) and stretched them there. It did this fine. My version of PS (CS3) would choke on 32 bit floating point TIFF files but AP worked with it fine.
Once I had the S/H/O all stretched in AP I combined them into an RGB image. I used a gradient map for each filter to blend them as Red/Green/Blue.
Then I used Selective Color adjustment layers to tweak the final palette.
Overall I'd say for my processing AP does everything I need as far as the post-processing I would usually do in PS.
One negative I noticed is there is currently (afaik) no full screen preview in Windows. It exists for Mac but it uses MacOS functionality to remove the menu and window borders. The best you can do is remove most of the UI but the menubar and window decorations remain. I really like to work in a full screen mode for PS so that is something I would hope they address.
I found most operations were similar enough to PS that I think anyone proficient in PS could figure things out reasonably quickly.
Anyhow for $49 I would say if someone is starting out and already had something like APP or PI to stack and do some early processing then AP would be a good substitute to PS and avoiding the perpetual "Adobe tax" PS has now.