I'm not sure if anything about this has changed since I last tried ElementaryOS an unknown number of years ago, but one of the issues I remember encountering was that applications that weren't built-in had inconsistent styling. I think you get a better more consistent and featureful experience just by using KDE.
Even the list of applications on AppCenter is woefully limited, compared to using Flatpack and being able to access the full repertoire of Linux Desktop applications.
Isn't that just a Linux problem, where someone could be using GTK or Kirigami or Iced for widgets?
Most Mac developers use Cocoa/SwiftUI, so they get whatever widgets the UXEs at Apple have produced. Adobe famously did their post-2000 UIs in Java, so they have a nonstandard design system; you can tell they don't use Cocoa just by looking at their apps.
Since there's no Apple to make an official toolkit for Linux, each developer uses whatever he wants, which means this app might use the GTK Save button, whereas that one uses Kirigami.
You can tell the Adobe UI isn't native but it's actually pretty good. In fact, I believe they had the right call since Apple frameworks are too limiting for complexe software like that and they needed the cross-platform compatibility anyway.
Serif does the same with their suite of Apps and it's fine as well.
Blender is custom as well...
Apple aficionados tend to get worked up about it (and I used to be this type) but they need to understand that there is always a necessary power balance to maintain, and going all-in with Apple tools is not a very good decision for a large software. They have proven time and time again that they will just mess up stuff when it suits them and companies with such big software endeavors cannot submit totally to the Apple agenda.
And even as an end user I would argue that UI uniformity is not very relevant, how the software works / what it allows is much more important. Relying too much on Apple software is also a losing strategy, as they have shown with Aperture and their office suite, they have basically rewritten from scratch every time with a newer incompatible format creating a lot of headaches and somehow managing to take years to give back functionality that existed before.
Apple focuses way too much on looks at the expense of many other properties (as they have shown again recently with the Liquid Glass) and its basically fashion at this point.
And since Apple cannot cater to every single use case, plenty of devs end up having to work up a custom UI on top of their framework, so if there is a complexe use case, you might as well cut the middle man from the get go...
Even the list of applications on AppCenter is woefully limited, compared to using Flatpack and being able to access the full repertoire of Linux Desktop applications.