I installed pm on my nexus5, which was quite easy actually.

Here is what I found:

I love the "oh, it's georgeous" when pm starts

It is really nice looking, and the plasma part appears to work well, although there are glitches.

Networking is possible, but it is not stable. Sometimes it does not appear to work, a reboot might fix. Clicking the checkbox to enable/disable the wireless does not work all the time.

No working webbrowser. Firefox crashes on startup, chromium crashes when I press return after entering a url in the tiiiiiiny address bar.
All non-kde apps fail to understand the high-dpi display. UI is practically useless, I needed a magnifying glass to read the firefox crash dialog.
terminal and console is working. The "resize-root-partition" command is not found. Maybe some of my problems is caused by that?
I wanted to try and add my nextcloud account using the "owncloud" preset of settings, but the dialog is jammed, so unusable.
ssh (and scp) to phablet@10.15.19.82 does not work. ssh just hangs.
I managed to screw up settings, it can not start right now. After (another) reboot? Yes. Happened again, after trying same operation.
Looks like SMS is missing - that is what I need the most to actually consider trying to use it.
I was not able to start peruse, whatever it is. Just hangs.
Screenshot (spectacle?) does not work. It starts, and shows a blank screen apart from a "Take new screenshot" button at the bottom, but pressing that does not do anything.
No phone sensors appears to work - GPS, orientation. Camera does not work.
Dolphin woks, but the UI is quite messy, with a too small sidebar and tiny toolbar buttons. Chess works, but apart form the board, the ui is not usable (dpi issues).
Discover can start, but not do anything, it shows nothing. Hm.

How would I go about testing apps, when there is no way to download, upload or otherwise retrieve packages? Ideas to solve some of the issues mentioned would be highly appreciated!

Marble kinda works, it has the opposite problem of most apps - its ui and fontsizes are way too big. pinch-zooming is too fast/much, and very hard to use. Search works, but there are problems. I am not able to get it to display a route, although I did get it to accept a start- and end point. Both search and routing creates widgets in the map that takes up way too much space, this is not a viable design!