I have booked a RTW last year via a travel agent in Canada ... the difficulties (to me) was to try to change anything before departure - in my case, I wanted to change operating carrier of a later flight segment. I called the airlines who issued the ticket and the airlines who operated the segment which I needed to change, they all told me they couldn't change unless I have flown the first segment and told me to call the travel agent to change ... in my case, the travel agent was not responsive at all!! So, I ended up doing the change after using the first segment.

I prefer to book it with the airlines directly. I have used the online tool (2 weeks ago), put in dummy dates and CX Seoul had issued the eticket charging my credit card (non-KRW card) without any problem. I subsequently have sent emails to CX office in Seoul for other requests and they have been very very responsive, helpful and friendly!