The panorama merge function is one of my most beloved tool in Affinity Photo and I use it regularly to stitch together all kinds of photos.
However, the source brush still needs some improving. The precision is rather poor at times. It will produce a line sometimes 60 or more pixels away from the brush stroke rendering the whole idea of a manual selection irrelevant. Also, it's unreliable in it's selection, too. Changing the source at say the top left corner of one image makes the whole panorama rerender, causing to delete / overwrite previous brush strokes.
In general, I feel like the tool ought to "listen" more precisely to the user's input. Maybe adding the option to define lines in the panorama it should stick to (e.g. roads, rivers, horizon, etc.) would be a helpful addition, too.
It's just such a pain to be working on that huge panorama for hours, seeing all your changes reverted or deleted every few moments. Also, adding the option to go back to panorama source selection might be an interesting idea. I'd rather have a 3gb work-in-progress file for a day than to render the panorama thrice and redo everything again.