I really love the look of KDE plasma 5 and also the memory optimization done for the desktop, after disabling all effects the desktop becomes more usable and responsive, it's beautiful design and the platform has many features. I used to work on MATE for more than 5 years but now I tried to give KDE a try and after installing Kubuntu 19.10 I found many problems :
[*] Every time I start adding or removing desktop widgets the whole plasma desktop crashes and I should relaunch it again from terminal or force shutdown my machine.
[*] Sometimes when I close an app like artha from notification area the app is reported as crashed and kde-apport start to consume cpu.
[*] There is no way to disable apport even after setting 0 inside config file, the apport service is always started ! is apport used mainly by any kde apps ?
[*] Every app : ksysguard, Marble, Kig, Elisa, KOrganizer, Falkon, KDE settings, at some point crashes without any reason, I don't know why but this is bad, because other non kde apps like firefox, vscode, libreoffice writer... are stable and work fine.
[*] Sometimes Vaults does not accept my password, and I should reboot many times to get it working and accepting my password to access my hidden files.
[*] plasma-nm is generating infinitely the same message "plasma-nm: Wireless scan on "wlp7s0" failed: "Scanning not allowed at this time" inside .xsession-errors file
[*] I use dark theme but when running any app in root mode it's white, this really hurts my eyes a lot and gives some inconsistency for plasma desktop, apps launched with root should inherit current user theme.
[*] The mpd service is slowing down my boot for more than 1min40sec, it generates the following error in log "mpd.socket: Failed to create listening socket ([::]:6600): Address already in use", I removed mpd with cantata, I hope that there is no other plasma component that needs mpd to run.
[*] Some non kde (virtualbox, artha, gimp, inkscape...) apps looks bad with dark theme, and many times text could not be read on them.

I think plasma desktop should stop adding more tweaks and focus on fixing and optimizing current desktop, because it's great desktop but it gives me insecurity to use it for something serious, I'm always afraid the desktop will crash and I will lose my data !
With MATE I could run my machine for a week without any crashes, even the desktop was ugly but everything was stable and desktop never crashed.

I think you may have a broken or bad install, as most people are not experiencing these things, some of which are not related to Plasma or KDE at all.
Is this a clean install of Kubuntu, or did you install Plasma alongside of Mate? What was your specific method, if you did so?

apport, cantata and MPD, have nothing to do with Plasma, and are added by your distro (Kubuntu). You can uninstall them. Cantata is a graphical front end to the MDP Daemon.

Artha could have a bug where it crashes, or it could be a Plasma bug perhaps.

Your distro does not support having a root account, and 'sharing' settings with root is a personal preference that not everyone may share.

Gtk apps do not follow Plasma look and feel settings, which is not the fault of Plasma devs, they work hard to make Gtk stuff look better, and the Gtk folks do not seem to reciprocate (imo). but you are still able to change those applications by changing the Gtk theme in System Settings. I am going to bet you can't do that to KDE themes while running Gnome/Mate/Etc using Gnome or Mate's settings program. It stinks a little, but it is better than having to load a Gtk settings program for this purpose.

My computers run for weeks, months between reboots. While I was visiting Australia for three months earlier this year, I rebooted my laptop exactly once.

I do think that you have a bad install, to be honest, or something is missing. The crashes you see are not a common thing. I would suggest, if you do try a reinstall, to do the Minimal option, which will leave out things like MPD and Cantata. Else try a different distro.

In fact I did first install with fresh Kubuntu 19.10 ISO, then after playing with widgets and settings my desktop became unstable and all shortcuts were not working at all, then I reinstalled again the whole system, and I avoided playing with settings, I did nothing to my distro I didn't even tried to remove apport or kmail (which is not useful now that gmail is not working with it) or KRDC (which cannot access my windows 10 on my lan but remmina works fine), but my desktop became unstable and especially kde apps they work but from time to time they crash without any reason.

Edit : I think I added Kubuntu backport ppa to my apt, should I avoid it ? do you use it on your machine without any problem?

Yes, I use the PPA on my 19.10 system, which is an old-ish PC and I have zero issues. It is my Steam game system, mostly, and runs pretty much 24/7.
My main system is a new-ish laptop running KDE Neon

What sort of system do you have?

Operating System: Kubuntu 19.10
KDE Plasma Version: 5.17.3
KDE Frameworks Version: 5.64.0
Qt Version: 5.12.4
Kernel Version: 5.3.0-24-generic
OS Type: 64-bit
Processors: 2 × i3-5005U @ 2.0GHz
Memory: 5.6 GiB of RAM

How hard was it to file this under the correct subforum? Moving the post...