Everything is very good and safe. There is only one point: the driver and assistant smoke in the car, which can be somewhat understood due to the long distance travel.

We took a day time sleeper bus with HM Happycar from Hoi An to Phong Nha. I received a WhatsApp message from the organizer to ensure we found the bus ok before departing. The bus was extremely clean and in great condition. The little bed capsules were private and lay totally flat, or you could put them into a partially reclined position. I am 5'4" and could lay completely stretched out, my friend is 5'9" and could do the same with flexed feet. There was a USB port, lights and air vents. The driving felt very safe and there were two drivers who traded off which helped us feel safe as well. We stopped 3 times for toilet breaks. After getting dropped off and going about our day, we realized we forgot air buds on the bus. I messaged them via WhatsApp and they responded immediately that they would ask their cleaning staff. They found them and organised a nearby hostel for them to be dropped off at for us to pick up at our convenience. Very impressed overall, would book with them again.