I have fond memories of KDE 2/3 and GNOME 2. I also ran KDE 1 and GNOME 1 back in the days (initially GNOME cause of RedHat Linux and the license issues with KDE). If you compare them to Windows 9x and early NT they were quite good.
For example I do not think to this day there is anything tech-wise like COM with all of its related tech like OLE (which allows documents to embed objects exposed by other applications which themselves can run in 'embedded mode' to edit said embedded objects), ActiveX (which in a similar way allows applications to make use of reusable controls that can describe themselves), Automation (which allows applications to expose scriptable interfaces that can be called by any other application), Active Scripting (which allows applications to embed scripting languages which themselves are registered with the system so that, e.g., you can install a scripting engine for Python which then can be used to script any application that uses Active Scripting even if that application has no idea about Python) and a bunch of other things.
(note that DBus does provide the 'expose interfaces' part, but it is only about that whereas COM is a more generic technology that was used for all of the above)
[0] https://techbase.kde.org/Development/Tutorials/Using_KParts
ActiveX is just a standardized way to embed controls, in practice it isn't any difference than any desktop application that has support for plugins nor any less (or more) safer than that. COM in general is just a standardized form of a plugin system with a bunch of predefined interfaces (or binary protocols if you want) for applications that weren't written with each other in mind to still be able to work together and share functionality.
But the two main reasons i think KDE 5 is worse than KDE 3 is that as a UI it follows the same pattern as Gnome 3 (and Windows 10's "modern" apps, sadly) for wasting screen space everywhere and it does feel like it has a lot of moving parts everywhere which aren't exactly fitting perfectly with each other even though they are made for each other (i know that KDE 3 was probably also similar, but it always felt solid and stable - i wonder if breaking kdelibs or whatever it was called to a myriad of tiny little libraries that all applications and each other needing to be there anyway made things more brittle, though that might be my bias from whenever i try to install a KDE application in a non-KDE environment and i get a dependency vomit from the package installer).
I might sound too hard on KDE 5, but TBH if i absolutely had to choose between it and Gnome and had no other option (or the other option was a tiling WM - which i never liked), then i'd use KDE. It isn't bad, i'd still install it on a computer i do not want to fiddle around with much.