The Holy Orders available to Catholics are certainly useful in combating heathens when you can get ahold of but the problem is they're frequently bogarded, especially by Jerusalem and the HRE. The Hospitallers and Templars being in fief to Jerusalem sort of makes sense (they weren't actually vassals of the king) and likewise the Teutonics to the HRE. However, I think that one thing that would strengthen the Iberian kingdoms and prevent them from getting overrun by Muslims 2 out of 3 times is the inclusion of the Knights of Santiago for the Iberian crusades. I hope that Paradox considers this suggestion, it may be the answer to a big problem.
For more info on the Knights
http://www.chivalricorders.org/orders/spanish/santiago.htm
For more info on the Knights
http://www.chivalricorders.org/orders/spanish/santiago.htm