It's a buggy pile of shit, for starters. Constantly drops network connections, will frequently flood your network history with multiple duplicate entries for no apparent reason, will often mysteriously fail to connect to a wireless network for whatever reason even though others work fine (and other devices can connect to the wireless network in question without issue - even other NetworkManager-using ones)... that sort of thing. I know the underlying utilities (namely, wpa_supplicant) aren't the issue, since wpa_supplicant - in my experience - is much more robust when configured manually, with far fewer problems, but at the expense of having to edit wpa_supplicant.conf in order to manage wireless networks, which is a bit of a pain (though perhaps worth not having to deal with NetworkManager).
If someone figures out a way to port Android's network management system (I'm not sure exactly what it is off the top of my head, but I know it uses wpa_supplicant and that it's not NetworkManager) to general-purpose GNU/Linux distros, that would be a godsend.
OK I must be really lucky. I find NetworkManager just works. It does ask for my admin password when it doesn't need it but I find that if I just ignore that prompt then everything is fine.
The more common issue with passwords is that it incessantly prompts for the user's password or for the wireless network passphrase due to some strange inability to talk to whatever keyring daemon is in use (and thus an inability to retrieve network passwords). I usually work around that by disabling the keyring (either globally or simply preventing NetworkManager from using it).
If someone figures out a way to port Android's network management system (I'm not sure exactly what it is off the top of my head, but I know it uses wpa_supplicant and that it's not NetworkManager) to general-purpose GNU/Linux distros, that would be a godsend.