A QCGB is much easier and faster to use than change gears for cutting threads. Also, there is much less chance of losing gears.
But, if something goes wrong, the change gears can be much more straightforward to fix. I used to be a member of Techshop, a machine cooperative with a monthly membership fee. They had two Jet 1440 lathes. The gear change levers were very tough to operate and engage. Sometimes, the leadscrew would turn erratically, ruining the thread. This almost never happens with change gears unless the gears are terribly thrashed or something is wrong with the banjo. Eventually, the manager told me "this lathe doesn't do threads, didn't you know that?????" It took them several months (paying monthly fees for each month) to fix the lathe. At one point, there were several shafts on the workbench, all covered with rags to keep swarf off them. I rarely saw the tech working on them, but it was pretty intense. Looks like one little mistake and things would get all galled up. The tech was not in a good mood the one time I did see him working. The lathe did get fixed, and was able to thread for a while, but then broke down shortly after. In contrast, I bought a South Bend for a really good price without change gears. I needed to cut one thread, and I had the local public library print out the gears on their 3D printer. I installed the gears, and it worked smoothly the first time, and I was able to cut the threads right away, in steel. I would never have had the guts to tear into that complicated 1440 gearbox. And, it was super finicky.
But, if something goes wrong, the change gears can be much more straightforward to fix. I used to be a member of Techshop, a machine cooperative with a monthly membership fee. They had two Jet 1440 lathes. The gear change levers were very tough to operate and engage. Sometimes, the leadscrew would turn erratically, ruining the thread. This almost never happens with change gears unless the gears are terribly thrashed or something is wrong with the banjo. Eventually, the manager told me "this lathe doesn't do threads, didn't you know that?????" It took them several months (paying monthly fees for each month) to fix the lathe. At one point, there were several shafts on the workbench, all covered with rags to keep swarf off them. I rarely saw the tech working on them, but it was pretty intense. Looks like one little mistake and things would get all galled up. The tech was not in a good mood the one time I did see him working. The lathe did get fixed, and was able to thread for a while, but then broke down shortly after. In contrast, I bought a South Bend for a really good price without change gears. I needed to cut one thread, and I had the local public library print out the gears on their 3D printer. I installed the gears, and it worked smoothly the first time, and I was able to cut the threads right away, in steel. I would never have had the guts to tear into that complicated 1440 gearbox. And, it was super finicky.