I downloaded the Affinity Designer Trial yesterday. As a first test, I wanted to save a new version of a logo for a website. I opened the existing .ai file. Designer put it into a A4 Document.
There seems to be no way to simply crop the document. You have to select all your objects and transfer the dimensions manually to the document settings. Then you have to move all the objects around. In Illustrator, this can be done with the artboard tool in two clicks.
In the export persona of Designer, I noticed that it is impossible to snap slices to objects.
When exporting to png8