I had the same experience with Pencil, but switched to the desktop version of Draw.io https://github.com/jgraph/drawio-desktop/ which I really like and recommend.

draw.io is great for flowcharts and similar diagrams, but every time I've tried to mock up UIs with it I quickly find myself spending 90% of my time fighting snapping and being unable to do precise movements. Perhaps it's just me not knowing how to use it properly.