Why it stays safe

Your saved bars are stored as real browser bookmarks, so your account syncs them to every computer you sign into. The trick is that switching never moves those saved sets - it only copies one onto a throwaway toolbar - and the choice of which bar is currently active is remembered per machine, locally (it is not synced). Because nothing mutable is shared between machines, browser autosync has nothing to collide on.

Other switchers move bookmarks in and out of the toolbar, and the toolbar itself is synced - so two computers fight over it. A common failure: you leave PC 1 on "Bar 1" and power it off; you work on PC 2 with "Bar 2" showing; later both sync, and you end up with PC 2's bookmarks dumped onto "Bar 1" while "Bar 2" sits empty - a tangle. The copy-based, per-machine design here is exactly what avoids that.