I find a few problems with STI:
- Many little parts that can break on the road. Not a real problem for commutes, errands, day rides, etc., but if you travel far from bike shops, it might be a problem. Many tourers therefore carry a set of downtube shifters "just in case".
- STI works in indexed mode only. For the rear end, it means one absolutely needs 8-speed shifters with 8-speed and 9-speed shifters with 9-speed. Non-indexed is also slightly more reliable in crappy weather (I cycle throughout Winter) and can work even with a bent derailleur, like after a crash (could be a problem while touring). BTW, working in friction means I never adjust cable tensioners.
Front bar-end and downtube shifters work in friction only, which means I can use whatever rings I want without any shifting issues.
Bar-end and downtube shifters may be shifted from one end to the other; there is no need to "click" one's way through the gears.
Non-STI brakes also mean that my brake levers only move one way. A minor preference, I might add.
Finally, preferences depend where one rides. I place my bars a bit high, but ride almost exclusively from the drops. That means my bar-end shifters are actually closer to my hands than STI.
Regards,