I recommended Semco for my friend's new teak deck furniture, based on the recommendation of a trusted FL sailor I know. I finally got to see the furniture after a couple months, and I felt it was due for another application. I don't see how it would build up. It's a watery finish, and you should apply it frequently, but it goes on so quickly and easily it's not a problem.
Cetol will build up. In my experience, on new teak, it bubbled after maybe 2 seasons. On the old teak that I sanded, it didn't bubble. Dang that stuff. Even though I had wiped down the new teak with acetone, apparently not thoroughly enough... It'll get scraped down next spring, I guess, and maybe Semco. The other problem with Cetol is getting the right color. I will tell you that Cetol Marine Light has a translucent orange color to it, that I don't care for, but I have become used to. Cetol Marine Gloss is colorless. I forget whether or not Cetol Marine or Cetol Marine Natural Teak is the closest to a varnished hue. One of those colors is a pretty dark brown, which probably isn't appropriate for teak on a boat - I mean, if you like the look of varnished teak. Cetol goes on looking like translucent paint - a thin film finish. It does not build and fill the grain the way varnish does.
There are other products out there, such as Honey Teak. There is a well respected sailor here who posts excellent sail trim advice, and he loves Honey Teak, says he gets about 6 years out of it. It's expensive. Near as I can tell, it is some form of epoxy or polyurethane.
I've also heard of Bristol Finish, which is a 2 part polyurethane finish. And Interlux and Pettit also make 2 part poly varnish-like finishes.
One product I know of is Coelan. It's a 2 part finish, that is supposed to have 300% elasticity characteristics, so that it gives with the expansion and contraction of wood caused by moisture cycles. I've never used it on wood, but it does mate well with the stretch of wet ballistic nylon we use for skin-on-frame kayak skinning. Again, it's very expensive, but in my group of traditional Greenland kayak builders, it's regarded to be the best, longest lasting skin treatment. Given its expense, most opt to use Coelan on a frame you know you LOVE after the first skin has worn out. I do not know how well Coelan will handle the oils of teak.
We cruise full time.