Have now driven both, as well.
From Nang Rong area to Ubon Ratchathani, strongly prefer 24, except for the last NE/SW section. There, it has 2 lane and road construction. 226 is still a work in process.
Not really a good connector between 24 and 226 except Nang Rong/Buriram
Was very surprised to learn of direct UBP - BKK flights @ Ubon airport.
I have driven thousands of KM in Thailand, north to south and east to west. Its an experience and to me a welcomed one to see this country. I agree that the 226 is a work in progress and the 24 has another two years togo to finish and that's just to Mueang Det then you have another 40 km of rebuild from Mueang Det up to Ubon and that will take another 2 years. As for flights from BKK to Ubon they have been doing that for years. My wife friend that we pu at the airport flew Thai Air from Malin to BKK and then Thai Smiles from BKK to UBON. BTW I did get 2 premium bottles of Italian wine from her husband winery.