I don't know about Lewis and Harris but have lived on Benbecula, which is connected by causeway to north and south Uist. You can drive on every tarmac road in a day, there are a few rough tracks but most go to someone's croft, or to peat cuttings or to a beach/ sea loch. All will be short you're on an small island , even Lewis and Harris they will be short. A convoy of outsiders may well upset the locals.
Oh the views are pretty good wherever you are on the islands even on tarmac.