I wouldn't say that, but you have to move quickly. I've used stealth to start a lot of the assaults. It can give you a real tactical advantage by letting you sneak into the settlement before the fighting starts so you're taking out the guards and turrets and such from the inside, which is a lot easier. You just don't want to meet up with your warriors before the assault starts. All the time I've stealthboyed into a settlement, picked a good area with lots of cover, and then started sniping people. Once you fire the first shot, your warriors will either come or spawn near you, so the stealth is out the window at that point, but having good stealth can be pretty decisive. If you're good with explosive, even better because you can sneak in and throw a bunch of grenades or lay mines.