I'm not so sure that there is a 'being too Joint', but, possibly, not being ' RAF enough'. A good friend of mine went Joint about 15 years ago and we were talking about this the other day. He, like me, did a series of Joint, Policy and overseas appointments and did pretty well in them, but it was, in his case, a matter of 'out of sight, out of mind'. OJARS written by a non-RAF CoC (and worse, a non UK first RO*) just don't seem have the same gravity of Light Blue ones; silly really, when roughly 30 of the RAF is outwith of the traditional RAF environment. In my friend's case, and in spite of an extensive Service, operational, Joint and international experience, colleagues who 'have stayed close to their desks and have never gone to sea' in a series of dull, but visible, staff appointments have been promoted above him
I am minded of my own situation about an OCR (pre-OJAR) about 10 years ago when the Desk Officer of the time asked rather naively 'what rank is an Ambassador?**' and then insisted I got a light blue officer in the reporting chain. Some random Wg Cdr I'd never met wrote some drivel about him as 2nd RO, clearly trumping what the Ambo said; nevertheless I shouldn't complain, as it got him promoted to Wg Cdr.
* A non-JPA OJAR - an NSAR - strips the non-UK 1st RO of his or her rank and appointment when it is entered on to the system, so the Board have no real idea who wrote the observations and made the recommendations on the subject officer.
** FYI, an Ambassador is normally regarded as a 2*.