User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-50.80.96.33-20130222215852/@comment-3228423-20130222230205

Some of the Reapers' advantages aren't even advantages. Indoctrination fields only work in close proximity to a Reaper or anything made by them (meaning they have to be inside the things for it to even work) and their FTL around 30 lightyears a day. The Forerunners can travel around 10 lightyears an hour.

And as stated below, they have be able to beat the Forerunners or the Imperium in order to adapt to their technology.