One, they're hard to see and blocked by line of sight (voice isn't.)
Two, they require hands to be free to communicate (not always the case, and in situations of survival it's probably not doable at all.)
Three, they're limited versus voice. Voice has soft and loud and more sounds can be made than variations of hands. More meaning can be ascribed to a sound.
I agree with your first two points, but I have to disagree with you on this third point. Mute your computer and check out the way she provides emphasis in this[1], or the way Lydia Callis signs so expressively[2]. Large motions are like loud exclamations, smooth, slow motions are like a crooning voice, etc. People who sign have a "voice"--a style of signing. Signing without any sort of emphasis or expression is the sign equivalent of speaking in a monotone, and can actually make it harder to understand someone, because these elements communicate a lot of meaning, just as intonation and volume communicate meaning in speech.
The second doesn't seem to come up much in practice. Can eat and sign at the same time unlike voice.
Third isn't true -- signing is more expressive because it takes advantage of space, facial expressions, and signs can be modulated to adjust the meaning. But it almost has to be since the symbol rate is lower than speech.
If you're being attacked or in a fight, do you really think you'll have a hand free to do sign language?
The article convincingly argues there's more to this than just your list of problems with hand gestures and the article