The reason that people (includig me) say travelling at night should be avoided is the additional danger of not being able to see in the dark, and of having a tired driver. The general driving standard is pretty poor, and at night you have to contend with extra problems.
Your bus will be sharing the road with pedestrians (no pavements) animals (no lights or road sense) and other vehicles, often with no lights, both moving and parked. Your bus driver will almost certainly have been up all day, maybe he has chewed enough miraa to keep him awake all night too, but who knows.
I heard this years ago, and it's something to consider: if you see one light ahead, it may be a motorbike or it may be a big truck with only one light. If you see two lights, it may be a big truck, or 2 motorbikes carrying a wardrobe between them.
I would never take a night bus anywhere in Africa unless there was no other option, but it's your choice.