Some features are “missing” or don’t work in a similar way. For example, Affinity Designer doesn’t have shape replication tools like Illustrator, manual copy paste is required. You also can’t trace an image to turn it into vector outlines. Just two things off the top of my head that I noticed because I used them extensively in Adobe Illustrator. So if you’re only using a subset of features you’re probably fine, but without testing Affinity’s products for yourself it might be hard to tell if they’re a like for like replacement for you.