NXT for example is easier because it doesn't store the funds or addresses on a wallet file, instead, it's a brainwallet that is kept in the network itself. An NIS would need to safely store the data on a file, thus needing to sit on the server. Unless the online wallet site sports a SSL certificate, it could lead to funds being stolen (for example). Every private key is yours to keep, but even downloading / uploading those are a potential hazard.