The OU and other certification agency's problems with the Triangle-K are well documented with in-person interviews in Professor Timothy Litton's book Kosher: Private Regulation in the Age of Industrial Food.
Selected excerpts (from page 82 to 84):
Triangle K has much in common with its larger competitors ... There is, however, one curcialcrucial difference: there is a widespread consensus that Triangle K certification is unreliable.
...
Critics of Triangle K argue that Ralbag's kashrus standards are below the industry norm. One industry expert, who wished to remain anonymous, explained as follows:
He goes on with an example of using Kosher animal fat and vegetable fat on the same equipment but calling the vegetable fat pareve without Kashering. Ralbag says that his father did that, but he no longer does.
Nevertheless, this example reflects a belief that Ralbag has kashrus standards ... that, while principled, are sufficiently outside of the mainstream that other agencies will not rely on his certification. As another kashrus expert explained, "It's permissible under Jewish law, but it's a standard that many people are not willing to accept."
Critics of Triangle K also accuse Ralbag of lax administrative practices.
He describes the Triangle K not inspecting jointly certified facilities with the OU - they only ever see the OU Rabbi there, and having insufficient administrative capacity to know and track what he is certifying.