Claudius, I think you are right on in your assessment that this really damages Triumph's reputation. As someone who spent some time as an editor and road tester for a motorcycle magazine (San Francisco's "Street Bike/Independent Biker") in the late '90s I think I can speak to this somewhat.
IMO, Triumph just doesn't have it together when it comes to their press fleet. The bikes are often delivered to magazines with very poor preparation. I once received a Thunderbird Sport test bike from them that had over 6 quarts of oil in the engine, and the coolant was overserviced as well. As a result of the excessive oil in the engine (which was not noticed until the bike had been ridden several hundred miles) the engine was constantly blowing seals and leaking oil. When a British company is trying to cement a solid reputation with American riders, the very last thing they need is a magazine reporting oil leaks.
Likewise, the reputation of the TT600 was scarred from day one when the first test unit at Motorcyclist magazine fragged its engine. The cause was a foreign object that got ingested into a cylinder through the throttle body - clearly someone preparing the bike left a loose bolt or something in the airbox. It was a dumb mistake that could happen to any brand of motorcycle, but a couple hundred thousand Motorcyclist readers saw, "The Triumph's engine blew up," and the damage was done. There are other examples.
The sad thing is, my direct experience with Triumphs is that they are very well engineered, on par with anything from the Japanese manufacturers. If Triumphs are usually a step down the performance ladder, it's often because they chose reliability over bleeding-edge performance. Again, I point to the TT600. I've never heard of another failure like the one at Motorcyclist magazine, and there are now many TTs out there with 30k, 40k, and even 50k+ trouble-free miles on them. It has proven itself as an incredibly durable motorcycle.
But, these problems keep happening to magazine test bikes. Is it bad luck, bad press-fleet preparation, or something else? I doubt that it's because the 650cc displacement presses the limits of reliability on the engine, however. It is generally believed that Triumph initially planned a 750cc variant of the TT, but scrapped the idea when it became clear that buyers wanted big Triumphs to be I-3s, not I-4s. The engine itself is pretty overengineered for a 600 IMHO.