From the title, you an probably tell I'm a Sketch person. I truly love Sketch, but it's pretty buggy and limited, so I was very happy when I ran across Affinity Designer.
However, as an app designer, I commonly use 3 features of Sketch that are not in Affinity Designer:
First of all, I love infinite canvases. I feel like limited sizes always end up having redundant or too little space, causing me to constantly resize it to satisfy my OCD. I also almost always only use slices (similar to slices and art boards in Sketch) for my graphics, since I like to develop a lot of elements of my app in one document. However, this only works well in an infinite canvas or, as I said, one has to keep readjusting the canvas size.
Another thing that I commonly use in Sketch is the background blurs feature. It is very common in iOS design to see a background blur and it's very useful to be able to easily re-create that with the click of a button in a vector designing software.
Finally, to be blunt, the slices exporting in Affinity Designer are dreary. First of all, they only allow bitmap export formats. In iOS, I typically use PDFs or SVGs for web development. Therefore, being limited to bitmaps is completely useless to me. I don't want to create a new document for every item and export them individually that way. Another thing I dislike about the Affinity exporting is the lack of flexibility in export sizes. You simply limit us to 1x or 2x. This makes slices completely useless for app development. Developers commonly re-use images at different sizes within an app. So, for instance, in Sketch, you can type in the obvious "2x, 3x, 5.5x, 42x" whatever to multiply the size by that number. However, you can also type in something like "32w" to scale the image down to have a width of 32 pixels. The same works with height by using an "h" behind the number. I strongly suggest Affinity Designer improves exporting, since having to create graphics in Affinity and exporting them to Sketch is quite the dreary process.