I'm planning on going bike touring in S.East Asia. I've been planning which bike to get.
Companies like Thorn offer custom made bikes (specially designed for adventure-style cycle touring), but they somewhat expensive and I have enough expertise to build something similar myself.
I was thinking to use a mountain bike frame because they are designed to take more abuse than road bikes and I plan to ride on rough tracks.
A mountain bike seems to offer much of the ruggedness offered by a custom made bike, but then not being a bike designer there must be some shortcoming using it to carry luggage and over long distances.
I know people that have done extensive touring on a mounting bike.
You have a sturdy stable frame and gear ratios for climbing.
Look for a steel frame that has lugs for racks.
And a solid fork over suspension.
The disadvantage is not quite as efficient.
And the single hand position is more fatiguing.
You certainly can use a mountain bike for touring, and many people have done so. They have suitable gearing for climbing and are generally stout enough to support the weight and abuse of loaded touring (caveats below). However it is worth noting the differences between a mountain bike and a touring bike, and consider these factors for your trip:
MTB frames usually have a more responsive geometry with shorter chain stays and wheelbase, steeper steering angle, and a higher bottom bracket. These features make a MTB more nimble on trails, but less stable when loaded and traveling at high speed.
MTB frames are usually aluminum which often doesn't provide as much vibration dampening. Aluminum is also more prone to structural fatigue than steel and with many miles can crack or worse. Carbon fiber is even less suitable than aluminum and should never be used for loaded touring for safety reasons (until a carbon frame purposefully built for touring is available).
MTB frames with rear disc brakes often do not have rack mounts; the usual location of a rack's lower connection point conflicts with the brake calipers. Old Man Mountain makes touring racks which fit bikes with this limitation.
MTB frames usually have fewer water bottle mounts.
A MTB with suspension, unless locked-out, will decrease peddling efficiency. Unless you're touring on very rough roads the suspension is just added weight.
A MTB's straight handlebars do not provide different hand positions for avoiding hand and arm fatigue, and potential hand nerve damage from extended compression of the ulnar nerve.
Smaller wheels, such as the 26" wheels commonly used on MTB, are stronger than larger wheels such as the 700c of road and touring bikes. These wheel's replacement parts (tires, spokes, and rims) have also traditionally been easier to find in less developed countries.
MTB tires with aggressive tread can have lower rolling efficiency than slick tires, and at least contributes to a noisier ride. Tread can, counterintuitively, decrease the traction of the tire on a smooth road.
Despite these differences, a MTB can be a very suitable touring machine. Some of the best touring options are early (1980s–90s) rigid (no suspension) steel frame mountain bikes, which can be found second-hand very inexpensively. Some MTBs may even be converted to use drop bars with road brake levers and bar-end shifters (you'll want to get an expert opinion from a local bike shop as some MTB brakes or derailleurs aren't compatible with road brake levers or shifters).