Bueng Khan to Nong Khai: B+. An enjoyable drive with some stopping points. Scenary in parts but a lot of it not so much. Traffic flows well. Road in good condition mostly.
Nong Khai to Chiang Khan: A+ most of the journey is scenic with countless great stopping points for food, drinks, photos or rooms to stay. I'd be happy to do this drive anytime.
Chiang Khan to Uttradit: B. this drive is rather nice but very limited on places to stop. Some roads lots of potholes. Still enjoyable though.
Phrae to Nan: B+ mostly good quality highway road with several lanes. Lots of strawberry farms. Nice scenary. Enjoyable drive.
Nan to Pua: A+. I took the middle road. There are 3 options and the middle road looked the best option for both time and views. Some outstanding views, great restaurants and stopping points. Not much traffic. Really enjoyed it.
Phetchabun to Chaiyaphum: B+. Some really nice views here combined with decent roads and not much traffic. Not many stopping points is the negative.