Are drainage holes needed?

[Ray]

Exactly. "Semi-Hydroponics" is not a complete definition of "orchid culture"; it primarily addresses medium and watering/feeding. Temperature, air movement, and humidity are still part of the overall equation, and how well they match up with the conditions within the S/H pot to meet the needs of your plant is critical.

[daddydoall]

Most of my Cats are doing well in SH.
Just a few have failed, like Millerii, angerii, some minis but those like LECA if allowed to dry out.