Kdenlive by KDE is a fully featured open source video editor that works on all major platforms. KDE team is working hard to make it more acceptable and powerful by adding more features. It has all the basic and advanced features that other free video editors have already, but it is open source and you can fully trust it.
KDE dev team is adding some features as the other popular video editors. But some of those features are not easily discoverable. One such feature is unavailability of thumbnails on Kdenlive's video timeline.
I myself struggled a lot and didn't like Shotcut for the exact same reason.
After digging a little bit, I found out that all frames can be enabled in the Kdenlive's timeline preview.
By default, Kdenlive only shows first and last frame for a video. This can be a little annoying for some people. Besides, all frames in the timeline preview can help you jump to specific part in the video without playing it all.
Luckily Kdenlive developers listened to the people, and they added the option.
Here's how you can see full preview of video in Kdenlive timeline.
Now, you enjoy the video editing on Kdenlive with all video frames available in the timeline preview.
The full preview of the video frames in video timeline will allow you to select and cut video segments very easily.
If you'd like to see the video tutorial for the same, there here you go.
This video will show you the above quick steps in an interactive way.