The theory of the curved “L” foils is very sound (and apparently also in practice as well); the primary problem with them is one of manufacture/construction. There are a LOT of side’s way leverage loads on the vertical part of the foil even under quite small sailing speeds and this alone creates difficulties in constructing a rigid/strong rudder, particularly one that has a high aspect ratio. The “T” foils on the other hand, have only minor sides way, leverage load differences acting on the vertical foil (they tend to balance each other out)
The main reason that I could see for using “L” foiled rudders would be where there is a problem with the “maximum overall beam” of a cat where the T foil would be wider than the allowable beam.
This in itself doesn’t have to be a problem if the T foils are “raked” aft to get the desired surface area and still not extend any wider than the outside of the hull.
By extending the foils back there is also a “hidden” advantage that, as the foils move up and down they also generate a small forward “thrust” to the boat.