At this point, it may be best to view the Affinity suite of products as potential companions to Sketch, rather than competitors. In fact, using them together could allow for many to replace or avoid an Adobe CC subscription.
While there are overlapping features in Designer and Sketch, if you are already comfortable using Sketch for UI/UX design, you might appreciate using some of the features that are unique to Designer. For example, it’s a great for creating organic illustrations because it supports pressure sensitive tablets and offer many textured vector stroke effects.
While Affinity Photo is a great competitor to Photoshop and offers the raster image editing features outside the scope of Sketch’s toolset. There are plenty of more in-depth articles on Affinity tools, here’s a good one. The main point is that even if you were to purchase them in addition to Sketch, you are still looking at noticeably smaller investment compared to Adobe CC.