You need a bike you feel comfortable on, so if it's your own old mountain bike you already know how it handles.
There are no real cons, many people travel on mountain bikes. The bike may miss things specialized touring bikes have, like a front fork with threaded holes to mount a low rider on. But p-clams work as well for this, and you could always buy a new fork, eventually. Same goes for fitting a back carrier.
Pros: 26 inch wheels are strong, tyres for it are readily available all over the world. You already have the bike. Mountain bikes have the low gearing you might need for long climbs on a loaded bike.