What is strange is assuming that Parthian were mostly horse based, and then arguing that horse breeding was not possible except in steppes.
There are a lot of thigns wrong with Parthia, including the royal domains restrictions based on a hidden generic ressources instead of a specific parthian related cultural ressource.
Beeing limited in our own territory and having to invade a non-iranian country just to have more opportunities for royal domains is strange, whatever the gameplay consideration behind it.
Historically it just doesn't make sense.
The same goes about the "usurpator" trait based on the tribe of the general administrating a province. While this is a good idea, it doesn't fit in the gameplay where you just can't chose a province leader when you have more than two, and if you have too much leaders you just can't manage anymore.
I won't even talk about the fact that when you king is not an Arsacid, it becomes a pain just to find a way to fill a royal province with a royal familly, even though a Suren or a Karen could creat technically a royal dynasty of it's won. (at the end, you end up with a Karen king, a Karen prince who are tagged "royal" and then all the other Karen who are not, while arsacids are royals even though they are not part of the king's clan anymore...)
I am really sorry but Parthia was the reason I started to play EB, and I was really enthousiast during the EBII construction and played Parthia a lot, but I waited a lot and all I saw was more complexity with less historical accurate features and less practical gameplay features.
I don't say that to destroy your work of course. Nothing is perfect, but anything to make Parthia better is worth saying.