I also found ISS very helpful, I think United World College is prefered by many because of the facilities and I feel it is very academic and elitist. I have three children to fit in at different grade levels, all have different strengths and I don't feel that United World will suit them all.
Two of my kids are middle school age, it is not uncommon for
International Schools that offer pyp and IBdip not to offer MYP, I believe it is considered the weakest of the IB curriculums and it wasn't a pressing concern for me when chosing a school, that my youngest finishes the PYP curriculum is important. IB DIp doesn't suit every student and schools should offer an alternative for those that don't follow the IB route, some schools only offer IBDip and so have strict entry rules to ensure they only get students able to complete the programme.
OFS and ISS are next door to each other, OFS has more facilities but I'm told they are not adequate for the large number of students. ISS has very limited facilities and so I believe travels off site when it needs facilities.
Thats all I can say really, my opinion is based on research and communication with the schools and people previously connected with schools, I haven't visited any of them and so do not have all the information I'd like.