Ignoring for a moment arguments about the relative nutrient density/calorie density of tubers vs grains and how they might affect insulin levels there is a growing body of evidence to support the hypothesis of Richard Wrangham that it was the consumption of cooked tubers that lead to the increase in brain size and development that created modern humans and not our hunting ability. Therefore as we've probably been eating roast tubers for close to 2 million years our bodies have had plenty of time to adapt to them unlike grains which we've only been eating on a regular basis for around 10,000 years.
Did Cooked Tubers Spur the Evolution of Big Brains?
Digging Chimps Provide Insights Into Early Human Diet.
As far as I'm concerned roots, shoots and tubers are ideal foods for people who are concerned with weight/insulin control as they tend to be extremely nutritious while having relatively low calorie densities compared with grains and due to their high water and celulose content they're very filling which makes it far less likely that you'll overeat.