Hello,
I've run into some limitations in Affinity Designer.
Affinity Designer (I haven't tested AP) doesn't seem to resize elements (and documents) at the rasterization stage (i.e. when rendering it to the screen), but it actually changes the properties of the elements in order to resize them.
Example: I use a vector brush stroke with maximum stroke width (100 pt) in order to paint on a certain design I'm working on. I decide to resize the canvas to double the initial resolution (with rescaling). However this particular shape doesn't get resized (or rather, it becomes bugged) because AD attempts to change the stroke width attribute of the element to something higher than 100 pt and it fails (it no longer renders properly). DPI changes don't seem to help this issue either.
The same thing happens when resizing certain elements, such as Artistic Text. The resizing operation doesn't resize the actual shape that is the result of the Artistic Text element, but rather changes the text sizes within the Artistic Text element in proportion to the resizing operation (dragging from handles). While I know this is done to create the best possible quality when rendering the text, it also hits certain hard limits.
And this also happens when working with layer effects such as outer glow width, etc. Resizing the document changes those parameters but they have certain limits (and they may use different units, pt vs px, etc.) (Bonus bug: undoing the document resize operation does not revert those values, and they become permanent and have to be fixed manually - I can submit a separate bug report on this soon)
My suggestion would be to apply a scaling factor in a separate pass (along with rotation, etc.) where it doesn't change properties of the element. This way it would also be possible to reset the scale just like how rotation or skew can be reset.
In addition to this, it'd also be great to remove limitations such as the 100 pt in stroke width or the maximum font rendering size, in a way that the document resize operations no longer hit hard limits with those properties where they have to be modified on the document-level.