Mechanical discs, even cheap mechanical discs, have come a very long way. I just test drove a rad rover fat bike with entry level mechanical discs. They stop well. And this is a near 70 lbs bike we're talking about.
My faves though are Shimano xt hydraulics. They're like buttah! A little less hand effort is required and modulation is so smooth and effortless.
A lot of people complain about Guide R brakes, but in reality, they work pretty well also. I'd say they're intermediate in effectiveness between xt hydraulics and mechanical discs.
Are they necessary on a road bike? They've become standard equipment so the question is largely irrelevant if you're buying new.