ludnid

The joke! Affinity Photo is sometime better than Photoshop, sometime as good as Photoshop and sometime less good than Photoshop, but it's look a real competitor.

It's like that. With the problem described AndyQ seems to back our feelings/impressions perfectly: after some time anybody working in pixel graphics will have realized that pixel masks ARE in fact just grayscale images applied in certain way to select corresponding pixels (meaning those having the same 2D position on the canvas) and allow to do certain things to them (i.e. adjusting their opacity etc.).
This given it seems only natural that masks should be editable just like any other grayscale image (including copying, pasting, painting in it, filtering it etc.) once you have them as the contents of your active window.
In Photoshop this is very much the case once you know how to make the mask content visible as the actual content of your active window. It doesn't take more than just going to the channels panel and clicking on the mask's channel – there you go and it's easy and intuitive. And turning the mask's content to a selection which you then can use in any pixel layer, adjustment layer, channel or other mask of your file is just one more click away...
In Affinity Photo this whole "field" of channels masks, selections and pixel layers is generally organized in such a "fragmented" approach that it becomes ”cryptic" as no obvious logic becomes apparent: so it becomes hard to understand and cumbersome to use. In effect you experience exactly what AndyQ said: you spend a day on a job which you could have finished in an hour with Photoshop. And true: as the way of achieving comparatively simple thing is so unnecessary complicated you're prone to have forgotten at least half of it of it by the time you have to work on the next job needing this a month later.
Please, to all you developers at Serif: I guess there are quite a number of guys here among us who REALLY WANT to work with your apps, who REALLY WANT you to succeed long term on the market. The workflow must become smoother, more unified and necessary actions for closely related goals should not be scattered over different places in the UI. I'd think in the end it just has to be at least as efficient as it is with those applications you – rightly – chose to challenge. Let's be honest: there may actually be a limit to the time professional users can afford to spend with Affinity Photo or Designer on jobs they could have finished in a fraction of the time needed if they had just reverted– however much loathingly – to Photoshop and Illustrator.

Exactly!!! Right at this moment I'm seemingly unable to adjust my mask's tones by simply making a Levels adjustment (which is no difficulty whatsoever in PS) or using Curves. In AP it just doesn't seem possible and I cannot see why this should be so, when I – just as Ron P. wrote – can paint into it and even invert it which all is actually nothing different than modifying the tones within the mask as a whole or in certain areas.
It seems to me that beeing able to control an finetune the grey values in a given mask in order to change the transparency of a masked object or layer is absolutel essential tro the whole business of masking.

I totally agree! I've been dabbling around with AP for months now and I really and honestly want to use it instead of Photoshop, but it's just driving me nuts every time when it comes to masking and editing masks (especially when you want to paste something into a mask). The process in AP to me is absolutely counter-intuitive and unnecessarily complicated. The whole clipping and masking business is disturbingly confusing in all the Affinity apps once you want to do something that's a bit more advanced than the simple basic stuff.
I don't think that making things different from the almighty competitor just for difference's sake is beneficial to the struggling users (and of all: switchers). You have to admit – however grudgingly – that the guys at Adobe did some things right in Photoshop (and in Illustrator and InDesign as well).

I regularly want to copy the background image into the mask, and edit it there .. in Photoshop it's copy and paste, as the OP says .. then you can look at the mask and edit it with any normal editing tool .. curves, levels, paintbrush ..
I actually can't follow your explanation above in response to the OP.. At all. Can you describe in simple, straighforward steps, how to do what I am trying to do? Yes, I have watched the tutorials, and if you can direct me to one that shows how to do this, I will happily put the time in on studying it.

Thanks, MEB .. but for those of us who work in a more painterly way with masks, you're making our point for us.
Our main point isn't that it's impossible to achieve the same result, just that it's a complex and indirect method compared to PS. Secondarily, I still don't see how your approach allows selective work on the mask. How, for instance, with your method, would you edit a mask using levels or curves, then use the history tool to paint those changes selectively into the mask .. something I do all the time in PS. Obviously I don't use colour-related tools on the mask, but I use just about everything else on mask layers, and almost always not applying the edits to the whole mask. I appreciate that there's always a balance between left-brain and right-brain processes .. the point is that Affinity imposes a very programmatic, left-brain approach, which feels heavy-handed for some of us, and which is also slower than the approach that PS allows. Why would I go with a program that makes the editing process slower and less intuitive?  Affinity is great for people who like to work the way that Affinity imposes, but for those of us whose basic approach to image editing does not comply, it's a struggle.
I can't believe that this is a technical issue. PS had this aspect of masking well worked out from very early days. I don't understand why Affinity wouldn't offer raster masks as an option when they're central to how many photographers work. When Affinity has a change of heart on this, I'll come back to it with rejoicing, until then, I'm out.