There are a number of possibilities, Jerry.
Each orchid has its own set of needed conditions, and "Semi-hydroponics" does not completely define growing conditions.
Probably the most common reason plants fail when transferred to s/h culture is that they are not treated properly in terms of getting them established - timing of repotting being they key, but also including managing the other potential stresses, as well. The plant has to grow new roots to accommodate the new conditions, so it may need to be "babied" while it does. Usually, moving from a very dry set of root conditions to s/h is the most stressful.
Issues with water and feeding can also play a role.
For example, if watered too infrequently, the chemistry in the reservoir can get pretty nasty - that's why filling the pot to the top and letting it flush the pot and reservoir is important, too.
Controlling the water and fertilizer chemistry seems to be a key, as well. Many say that most traditional orchid media have some cation exchange capacity (CEC) - a term more often used with soils that basically describes the fertility of the substrate, and its ability to hold onto nutrients to be "doled out" to the plants. Personally, I doubt that it's significant in most orchid media, but it is essentially absent in LECA. That means that the pH and nutrient contents must be controlled by the applicator, and it probably has to be controlled more closely than is the case with other media.
I'm surprised to hear about the paphs - most of the time they jump for the sky if accidentally dropped on an s/h pot for me. I'd guess that the oncidiinae intergenerics are simply less-stressed than they were previously - possibly the evaporative cooling of the root system - some of those guys get pretty picky about heat.
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