For some reason, Triangle K is something of the "Gadfly" of Kashrus organizatons, with a good deal of Lashon HaRah spread concerning the reliability of the Hechsher (see for example this interview and comments). I would assume that part of the reason for its poor reputation is its certification of the Hebrew National meat products, the only Orthodox supervision of non-glatt meat available on the market. (It is worth noting that R Moshe Feinstein was always a strong supporter of the Ralbags who are in charge of the hechser, and I have heard from a relative of R Reuvein Feinstein that he said that if not for the fact that he is makpid on Glatt, he would have no problem eating Hebrew National.)
I was always specifically concerned about its hechsher on fruit juices, many of which contain grape juice. Although there are some leniencies in regards to grape juice (it can be nullified in less then a sixth, and if mevushal it could be OK according to some Posekim), I have never seen any other certification on juices containing grape juice.
Marc Shapiro told me that he asked R Ralbag and was told that: "all the grape juice in HiC, Minutemaid, etc. is made specially under Triangle K hashgachah, in a special "crush" for these, companies. They make all the grape juice they need for the year in a month, under a mashgiach temidi.
I did not see this information anywhere else on the web so I decided to place it here.