I have spent at least $1M on Citi Aadvantage cards (mostly online) in the past 10 years and have never had a problem. I have been contacted a few times by Citi anti fraud dept when I have had strange spending patterns like me using the card in the Caribbean and my office in NY spending on it at the same time.
The only fraud I have experienced was when I used my Amex card with LatinPass/Globalpass.