They have had a small number of security incidents in the past however to my knowledge they made their customers whole. I also believe they had issues regarding wallet backups at one point however this was several years ago. There have also been controversy regarding their policy on what transactions to display on their block explorer and have had problems with their block explorer in the past.

IMO, the primary reason why some people say that bc.i is unsafe is because blockchain.info is a supporter of larger blocks (they have a different political position than those giving the warning).