I just pinpointed source of 99% of my frustrations with Plasma 5: - Plasma widgets.
Note: I am running KDE Neon (because Kubuntu is stuck with really old version of Plasma 5).
In my opinion these widgets prevent the Plasma 5 being the best desktop environment.

For example:

Clipboard widget: has stupid scroll bar. Klipper does not. Fix: hide clipboard widget and use klipper instead.
Volume control: missing "Event Sounds" from mixer window. Fix: hide it and use Kmix instead. Note: the biggest annoyance has been fixed in Neon: inability to use scroll to control volume.
Instant messaging widget: This is a pet peeve of mine. For starters completely useless with multiple accounts (no way to tell if one account is offline, no way to differentiate between contacts with same name on two different accounts, no way to rename contacts. Fix: hide it and install kopete.

Other annoyance with launcher is missing a lot of apps (eg: kopete is not in the list). Alt+F2 finds them fine. After going in and editing the launcher it seems to be there, so when an app is installed launcher list is not updated instantly.

It would be nice for calendar to show tool tip on public holiday, instead I have to click on the date. Regardless the current calendar is a massive improvement to what it was originally (as in actually displaying the public holidays).

So the question is: why doing all these rewrites and replicating functionality (badly) when there are already apps that work?