As a former user of Illustrator, there's nothing I miss from that app, certainly not the clunky Illustrator UI. (Unfortunately, Designer also has some clunky aspects to it's UI.)
Anyone, who wants to make up their own mind about the Affinity apps can download a free trial from their website - the apps work on both Windows and Mac.
Also, take a look at the 'spotlight' section of the Affinity website to see some of the work possible with the apps.
Because the discussion is not whether Affinity apps are good or not, but whether they are good relative to Adobe, anyone that wants to make up their own mind also can download a free trial of CC apps.
Where does it fall short for you? Out of interest.
One other thing that stands alone and is absolutely critical, that would seem to work in Designer but doesn't, is export for embroidery. It is a quirk of my local shop but there is no vector export format from Designer that my shop can use. They end up importing ("digitizing") a jpeg. This causes many more steps in the approval workflow. With Illustrator I just export an eps and they import that and it comes out perfect basically the first time. Obviously Designer can export eps but the software on the other side doesn't like it. (And even though their software can handle native illustrator, import of that doesn't work either. Has to be eps, from Illustrator, not Designer.) I'm 100% sure it's their crap embroidery software (I mean, they are using win95 if that indicates anything), but it is what it is. It's especially odd because embroidery is very low resolution. jpeg should be perfectly fine!! I just get way better final results with the eps flow.
That embroidery issue doesn't come into my evaluation of the goodness or not of Designer as a standalone tool. I just mention it because it does affect me and I think it's an interesting point to bring up. I'm sure many many people are dependent on Illustrator for similar workflow/pipeline issues. It doesn't make the software good or bad, but does create the dependence.
I think the issue people have is they get insulted when you say the software they enjoy to use is not very good. But there are 2 kinds of good. Absolute and relative. Relative to Illustrator, Designer doesn't compare. That is my critique.
There are some things I do like very much about Designer. Personas. Slices. SVG support is much better (keep in mind tho I'm stuck on CS6). These things, however, do not begin to challenge Illustrator.
I understand why Serif would want to stand apart with their own feature set, but now I find myself using 2 tools. Oh how I, and many other non-professional users, would love to replace Illustrator.
Keep in mind, I believe I am in the target demographic for Designer. I am an occasional user, not a daily use professional. The price and pricing model is perfection. The ease of use is perfection. I dare say that the online userbase/forums and free tips and help you can get are much much better. But the lack of features is saddening.
It's interesting because Serif (the company) does not compete with Adobe (the company), even though at a surface level they would appear to. The cost difference between Illustrator and Designer is worlds apart. Maybe this is why they don't work on feature parity. They don't want to be on Adobe's radar.
With Catalina, I'm soon going to have to bail on Illustrator. I'll probably have to invest some time in Inkscape. I could pirate CC or I could use CS6 in Windows VM (again, technically pirating although many would justify it), but I'm not that person.
I haven't explored the other ones as deeply but from what I have done, the features seem quite limited. For example, there's a translator for Photoshop plugins to work on Affinity Photo. Very very few plugins work with this translator. Why is there even such a middleware? Because the Affinity plugin ecosystem is very weak.
It's an alternative, not a "decent" alternative. It's the equivalent of Google Docs vs Microsoft Word. If you need something barebones, it gets the job done, and there are a few things it does better. But if you need just one feature of the better software, forget it.