I would ask if you've used an army of upgraded Janissaries? If the AI attacks you with a weakened unit and dies, you get healed. If the AI attacks you and fails to kill you, you kill it next turn with your 25% attack bonus and get healed. How is a self healing unit not a defensive asset? =p It doesn't matter if you see me first if my army is nigh unkillable.
Regarding America's +1 sight, I don't advance if it will leave me vulnerable to a counter attack, and in the event I need to scout I do it with horses, caravels, or even scouts. America's +1 sight is a nice to have, but any Civ can loosely simulate it with a single unit. It's definitely not as good as having an army of units that practically can't be killed save for the most unfavorable of circumstances.
Yes, Janissaries are good vs total morons, such as the AI. However, you can kill the AI with pretty much ANYTHING easily. They die easily to concentrated fire, flanking, anything from a later era, and are every bit as vulnerable to ranged bombardment as other units. They are good, but they are not game breaking.
Any unit you're using for extra sight, no matter HOW you are supposedly using it, can see 1 tile further still as USA. There are NO civs that can "loosely simulate" it, and when you're playing against something with an IQ above 50 or so, it is very helpful. Sometimes it is even helpful against the AI, as if you need ANY uniques there.
"nigh unkillable" is a total joke. You get caught by a knight in the flatlands, and you lose almost all your hp, and another one shuffling in can kill you. As you approach, your str can be cut down by archer type units, navies, or cities easily, making it hard to actually finish something off. Attacking a fortified unit on forests/hills isn't going to be favorable either - you'll be largely even against earlier things like longswords/knights (which don't get D bonuses but don't take the flat penalty there) and opposing muskets, subject to who can actually manage a surround on the other player. On defense, you'll probably get kills easily, but on offense against anybody competent, it won't be so easy to move in.
It is a strong unit, just as it was in civ IV. However, just as it was in civ IV, its benefit is mid game, where the American UA and most other civs' UAs/UUs/UBs have already been having an impact for much longer. What if you're fighting horse on horse vs an american player? You don't think it matters if he knows where your forces are even while you can't see him? Keep dreaming.
I'm not saying America is a top civ, just that it's ridiculous to put it behind a civ with such a flagrantly awful UA.