Odium's forces are far from the height of their power. For starters, the average Singer has a maximum of 1 year of military training vs the lifetime soldiers that the humans are employing. Given enough time Odium will have a properly functioning military not just generals. The other huge factor is war form, the average singer is naturally stronger, so strong they can leap chasms that require a Bridge that takes a minimum of 25 people to carry it for humans. They also come pre armored, in other words, once training sets in and they are not massively out skilled the singers are going to kick some serious ass.
While the Radiants are scaling and swearing higher ideals the Fused are not as weak as you make them out to be. For starters, they are not all on Roshar yet. At the end of RoW and in the middle we see new and much more powerful Fused and Regals show up. El is shown to be an incredibly dangerous enemy.
While their science is more advanced Raboniel is able to take most of what they have and with the way Fabriels have been described it's not that hard to make them. The understanding of what bits need to be stuck where is the hard part, once you have that down all you need is some gemstones and some metal. Their constructions so far are fairly simple and rudimentary the most complex being the airship but even that isn't that complex when you get right down to it.