The local scrub plus cloud render split is the part I would poke at first. I build a desktop video editor on my own, and the bug class that has cost me the most time by far is the preview and the export disagreeing. Two decode paths, two color assumptions, and a frame that looked right on the timeline comes out shifted in the file.
Mine broke on something small. A Display P3 tagged still looked right on the timeline and came out different in the file, because the preview path and the export path handled the color tag differently. Nobody notices until they open the finished export. How are you keeping the proxy scrub and the final render honest with each other?
Unlike other tools that bring video editing to the browser, we render locally and let you scrub your original footage! The proxies are for your teammates who want to see what you see, but they can also get the original footage through Kino's storage