Lack of world voltage is probably deliberate. Braun/Oral-B toothbrushes are usually cheaper in the US than in other countries (e.g. the UK) that use 220/240V. The UK-bought toothbrushes will work fine in the US.

I've had one US-bought base unit fail and, given the simplicity of the device (just coils for transforming voltage and induction charging) I'm assuming it's due to being plugged in to a 220V outlet for an extended period.