Budget travel in Southeast Asia and the question of when to take the flight vs the bus.

The decision matrix specific to Southeast Asia:

Flights you should almost always take: any journey over 8 hours where a budget airline option under $50 exists. The AirAsia Bangkok to Phnom Penh flight is 1 hour and costs $30-50. The bus alternative is 10-12 hours and costs $15-20. The 9+ hours of time cost at any plausible time value makes the flight correct.

Buses you should almost always take: any journey under 4 hours where the scenic route adds value. The coastal road in Vietnam between cities, the mountain pass between Chiang Rai and Luang Prabang — the landscape is the reason to take the bus.

The overnight calculation: journeys of 7-14 hours that run overnight and have genuine sleeper options (Vietnam overnight buses, Thailand overnight trains) are usually correct to take by bus/train because they save accommodation. The Bangkok-Chiang Mai overnight train is slightly more expensive than flying but saves a hostel night and you wake up in Chiang Mai.

The false economy: paying $15 more for a budget flight that departs at 5am from a distant airport requires an $8 taxi the night before and a $5 airport coffee and arrives at a distant destination airport requiring another $10 taxi. The $15 'savings' over a reasonable daytime bus can evaporate entirely in hidden costs and inconvenience.