The Minibus from the Oasisbar in Ha Tien to Kampot via Kep leaves at noon as I mention in this answer. Though the motorbike taxis leave whenever you want. And yes it is not as safe as in a car. Though the driver can take your backpack between his legs in front of him like shown in the picture.
So you either take a little risk or get the minivan at noon. 🙂 I took a motorbike taxi quite a few times and kinda like it.
Credit:stackoverflow.com‘