Since I don't think either SIN/HKG qualifies as "relatively undeveloped"
, thought I'd chime in about KUL.
KUL: If only 1 person, take the KLIA Express (RM35/$9) to Sentral and then metered-cab/subway to destination in downtown KL (cheap). If more than 1, it's more convenient and cheaper to take taxi (RM65/$17) directly to destination in downtown KL.
As for SGN, I opted for pre-arranged transportation ($4/pp) as opposed to figuring out the cab situation. Turns out, there are no highways in SGN and the 30-45min journey (for less than 5mi!) was congested, polluted and slow-going. A/C solves everything, of course
In HAN, cabs are $10 flat-rate into the city which seems like a rip-off compared to SGN but then again, HAN is 45-60 minutes (28mi) from the city - most of which is travelling 50mph on a dual carriageway (of sorts).
I have found that metered taxis are the way to go for airport transfers and getting around town except for when the taxi rates are not standardized i.e. different companies charging different rates like in Vietnam (also depending on the size of the car). Even then, with a little bit of research, taxis can be super-convenient and ultra-cheap!