Transiting China at least, Thais require a transit visa. I had a tha friend, on his way to the US, Visas and everything in order. He was not allowed to board the flight, because he didn't have a transit visa for china. He had to land and change flights there. He was in tears over it.

 

His sponsor managed to get him another ticket, flying through Singapore.

On 11/4/2019 at 11:13 AM, Boy Wonder said:

does the Thai citizen need some kind of visa to use this itinerary, or will they be fine assuming they are strictly only there for flight layover purposes?

For some countries, a resounding YES! Check it out thoroughly. For example a friend had his partner refused at check-in, for a journey to NZ,  as they transited/ changed aircraft, in Australia. Had no Oz Visa for her. He had to re-book on a non-stop flight.