Soldato
The thing about Gold is that it really requires a good group set up of both classes and people who can play them well (an adept who doesn't use stasis bubble is a bad adept for example) and people who either know how to work as a team or are on voice comms.
The worst part is it's just not worth doing when you consider how quick and easy silver challenges can be with an "ok" group.
I've had my fair share of failed silver challenges, depending on what class you play you can't always carry the group if they arent up to much(not that I havent let the team down before in silly circumstances). My success rate on gold is actually a lot higher than silver but most of my experience from gold comes from geth fb white farm runs which arent particularly taxing.
I only ever play in PUGs