In Unity3d the TGA files could be used not only to store alpha information, instead, you can use the Alpha channel to store Glossines, Metalness, Specular, etc values ( according to the shader), so it is quite popular to use this channel to draw and fix some important information, but after working with Affinity photo I found that: 1. I could load TGA files, but instead of showing the alpha channel as a simple Black and White image like PS, it show it automatically as transparency, so there is no way to adjust, paint or modify it easily. 2. In case you were able to do it, you cannot save it back again to TGA, so if you are making changes into existent TGA files for a particular project, there is no way to do it. You have to save the file in TIFF format and then into Unity3d or any other Game Engine, you have to make adjustments into your shaders linking back again the new maps ( accordingly to the project complexity this could take a while instead of just load the updated TGA files ).
I know that Affinity is a very good tool, in fact, I didnt cancel my purchase after all, because I was able to use it for almost all my regular tasks, but being honest, I think the full support of TGA files should be consider as a priority because the TGA format is a very popular standard into the game developers and there are a lot of assets into the dev stores that comes with that format, so having limited support, reduce the potential use of the Affinity tool.