Your experience at ASEF can be very different depending on in which department you work. None is better than the other, each one has its own strengths and weaknesses. The collaboration between departments is not the strongest, but you can yourself influence this, if it's something that is important to you.
Unfortunately Asian and European governments are not giving ASEF or the ASEM Process sufficient support (political, financial, human resources, etc.) which can lead to some challenges in relation to implementing projects. Expect hence having to come up with creative solutions to these and other challenges related to the characteristics of the organisation and its work.
While interpersonal conflicts have existed in the past, the situation has improved the past couple of years. Such conflicts can come up again though at any time (and in any organisation/company), so it is also your responsibility to contribute (and avoid being counter-productive) to a healthy work environment, if you get such an opportunity.