Even not being accustomed to bulletproof vests, the message it sends is all too clear: they are used to fighting unprotected.
Given Ferdinand started in Africa and most of his men were recruited there, this situation is easily understood. Yet, Yang Yi didn't underestimate Ferdinand's people because they might not have used bulletproof vests or night vision goggles, but that didn't mean they couldn't fight.
Of course, high-tech weapons and equipment are not limited to just bulletproof vests and night vision goggles, but these two items indeed serve as force multipliers and are more visibly apparent compared to other equipment like wireless communication devices.
If it came down to equipment, Trident was the strongest on the scene.
The gap wasn't huge, but Trident's night vision goggles were at least a generation ahead of the Fearless Mercenary Corps and one tier above Deyo's direct lineage.