When you think about it, undermount sinks make no sense. You've got a heavy item (the sink) which occasionally gets loaded up with an equally heavy thing (water), and often has an extremely heavy thing attached to it (the disposer, if we're talking about kitchen undermount sinks). With these stresses, you would naturally want to take advantage of any kind of support available. In the instance of a drop-in sink, you've got built-in support: gravity.
The sink lip rests on the counter, and this carries all of the weight.
But undermount sinks turn this all backwards. Literally. They are expected to press up against the underside of the sink counter, held in place only with epoxy and clamps. The sink moves a few millimeters out of place, and there goes your caulking. As you have found out.
All that said, undermount sinks are still great. They look good, simplify counter cleaning, and even give you a bit extra counter space. So, let's not condemn undermount sinks. Instead, we'll condemn the installer...
Why the Caulk Failed
If this were a DIY job, I'd say, "Well, it's tough to install an undermount sink. Not your fault." But you had a professional installer do it. That's his job. So you're right - he should have done a better job.
What went wrong? Probably two things:
- Cleaning. The bottom of the counter (where it contacts the sink) was not cleaned thoroughly. Denatured alcohol should be used to clean off all surfaces prior to caulking. Granite or solid-surface (Corian, SileStone, etc.) dust can often compromise the caulking and should be cleaned up.
- Clamping/Epoxy. Installation not solid. The clamps and/or the epoxy were loose enough to cause the caulk to pull away. Remember, they don't need to visibly loosen for the caulk to pull away.
It could also be that the installer did not use true kitchen sealant. Moen, maker of sinks and fixtures, recommends pure, "100% silicone sealant" for undermount sink installation.
While you may not like this answer, the best thing for you to do is crawl underneath and make certain that the sink is solidly attached before reapplying the caulk. You cannot do anything about the epoxy, but you can check the clamps and make sure they are tight.
Then, come up top again; scrape out as much of the existing caulk as you can without scratching things; clean surfaces down with denatured alcohol; and apply silicone kitchen caulk.