It's milder in the sense it has less caffeine than coffee and is also more approachable to tea drinkers because it doesn't have as strong of a darker roasted taste.
Mate has about the same amount of caffeine as coffee. More even because of how it's consumed. You refill the hot water repeatedly, each infusion drawing more caffeine from the herb.
True but also if I weren’t drinking mate I would probably drink at least 2 more cups of coffee a day, so all in all I think it comes out as less daily caffeine consumption.