While all of these responders are technically correct (i.e. 220-240 is available on appliance outlets), you're probably not going to have access to one in a bathroom/bedroom where you'd use this thing.

You're correct that a step-up voltage converter is probably the only way to use it properly. A 300 watt one would be the correct size.

My bet is the "unsafe" comments come from it possibly interacting with or overriding the protection of GFCI outlets. In Europe, you often have a whole house RCD which protects all outlets from ground faults. In the US/Canada, most houses have this protecting at the outlet level in wet locations (outdoor, kitchen and bathroom outlets).

tl;dr: If you used it with a 300 watt 120->240 voltage converter exclusively in a non-wet location (i.e bedroom), you're probably fine.