Wind typically dies down at night everywhere but not always. The fast kids in the 80H start at 4 pm and have two nights of riding but only one full day, but more importantly they are doing the windiest parts of the course at night.
One benefit to an afternoon/evening start is being able to sleep and if you are jet lagged, a 3 am wakeup call to make a 5 am 84H start is going to be rude. If your hotel is near the start, not so bad.
I think the big disadvantage of the 84H start is catching the wave of slower 90H riders. They are starting say 7-10 pm. You as a successful 84H will catch many of them and be right in the bulge. If you start at 5 am, most 84H riders will get to 447KM control, Loudeac, somewhere a little after midnight and the thought of tackling the hills into Carahaix or St. Nicholas won't be so appealing. Loudeac will be filled with 90H riders and a few faster 80H returning riders. Maybe 2,000 riders or more than 1000 for sure. If a 84H rider stops to sleep earlier at Tintineac or the special sleep stop between loudeac and tinteniac, you can't do a long sleep because you also have to make the intermediate controls. I do not have an 84H card in front of me but it would be
something like 39 hours to Brest and 45 hours for the return. If you get to Tintineac at 8-9 pm (360 km about) and sleep until 2 am, you will stay behind the bulge but you then have to pretty much ride the 240km right to Brest but you will have time. Loudeac to Brest is a hillier section. Personally, I think this is a reasonable strategy if you can wake and go at 2-3 am. Leaving at 5-6 am from Tinteneac is a risk. In 2015 on my return leg, I did see outgoing strong looking 84H riders between Loudeac and Tinteneac at 2-4 am. This is not a strategy for everyone but I think it is possible to mostly ride in daylight and have time to stop at cafes to eat, for the efficient and experienced randonneur. Some controls will start to run out of food, too. An advantage is less overall night riding but it can also be very warm during the day. It was into the mid 90's during the day in 1995.
The wind was ridiculous in 2019 except for the very fastest riders who reached Brest soon enough to have a tailwind. I was pretty slow due to sleeping problems but I got some tail utnil almost Carhaix. I even managed to top more than 60 mph on one hill with a helping wind. Then, it was a headwind all the way home. The 84H riders had the best wind conditions in 2019.....their last 24 hours, it was relatively benign.
In case someone reads this in the future, low temps on my Garmin in 2015 were 40F and 38F in 2019. It was much more damp in 2015 with condensing all over me. Damp in 2019 but not condensing on my cycling glasses. Dress warm. If you are from Florida or someplace like that, bring lots of warm clothing
Pauls lays the different start groups out well, I think anyway...
PBP Planning: What Start Time Is Right for Me? | Central Florida Randonneurs