I've been using Serif's Affinity Designer for about six months now, having been forced out of the Adobe ecosystem due to it being financially untenable. While there's still a lot of depth to the program that I have yet to explore, I feel there's some value in documenting what I've found to be its limitations as it pertains to the work that I do, particularly since it seems hard to find solid information on what Designer can't do.
For those unfamiliar with the product, Affinity Designer is a vector illustration app by Serif positioned as an affordable alternative to Adobe's Illustrator. It is available as an app for iPad as well as an app on the desktop. I use a Microsoft Surface so my experiences pertain to the desktop app on Windows. I understand the app / MacOS is the primary focus for Affinity, so certain things (like performance) may be expected to be better there than what I experience on Windows.