Worked with masons who swear by keeping the mix simple and just timing it right with the weather. Never saw additives make a real difference on their jobs.
My uncle's porch rebuild in '19 finally sold me. We used a liquid additive from a dusty bucket the supplier threw in for free. Three winters later, not a single crack has shown up in those steps, even with the freeze-thaw cycles we get. I was fully prepared to blame the mix when it failed, but the stuff just worked. Hard to argue with a finished job that stays finished.
Actually, that timing thing only goes so far. Good additives fight the physics that cause cracks, not just the weather. They can make a real difference when conditions aren't perfect.