A morning viva starts sooner, but an afternoon viva might finish more quickly.
An afternoon viva could be more nervous for a candidate because they have more hours to think before it starts.
But are there distinct advantages or disadvantages to the start time? Not really.
Your situation or your examiners’ circumstances might favour a particular time; anyone involved might have a preference. But the start time doesn’t make a great deal of difference.
When you know the date and time of your viva, figure out what you need to do well in the hours leading up to the start. Morning or afternoon, you have to manage yourself as you make your final preparations.