from my experiences with original Leitz external viewfinders in 21, 35, 90 and 135 mm I'd say that they even more inaccurate than the internal frames. The rangefinders are of a very high manufacturing standard but every small misallignment in the accessory shoe leads to a more or less serious difference between the viwfinder image and negative. With the Leitz 21 mm and the Cosina/Voigtländer 25 mm viewfinders this is more a estimation than an accurate composition. No external viewfinder can fully take into account the parallax mistake and the variation of the image field caused by fokussing from infinity to close range.
Everybody who wants to compose exactly into the frame should use a SLR with 100 % viewing on the screen like a F 2 or F 3.