On intl. multi-segment itins, agents are nearly always worth their money.
Thanks to web-engines becoming more imbecile monthly, it is nearly impossible
to book a multi-city itin on a website, let alone a RTW.
And while until 2003, I did book about 16 RTWs online, I haven't managed
to book a single one since.
The engines do not recognize through fares anymore, they violate married
segments so you get a different (very, very different) price in the very last
step long after the confirmation. They do not understand stopover rules.

In the 'good ol days', I could pick a fare - say on travel.com.au - construct an
itin and have the itin checked against the fare. I would get MPM violations,
min/max stay errors, min stops errors until I finally designed valid itin.
Completely impossible these days - if your need is not to travel from ORD
to SFO at 7 am an return at 4 pm, then you're lost with the webtrash of these
days .