To be fair, I have not used Trusted in a deck yet. I think it might be worth writing a review for why I don't normally consider it good option, though. I like the idea of Upgrade cards in theory. They are cards that are supposed to make your other key cards better, and I think normally that's pretty good. Of course, that does mean that you need cards to work well with them as they are not much use on their own.
The cards that work well with Trusted are Ally cards that normally have abilities that cause them to take damage or horror as part of the activation cost or have a ability that is triggered by taking damage or horror. Some examples would be:
If you play Trusted on one of these cards, you normally get one more use out of their ability, which can be pretty nice. Anything that adds value to these cards is a good thing in my opinion.
On their own, the extra points of soak are handy to have. In-faction, Guardian cards don't normally need a lot of extra health soak, but most will welcome the horror soak.
The reason I don't normally pick this card is because I feel like it has anti-synergy with Calling in Favors. Calling in Favors is the other card you want to run if you have a lot of Ally assets, but if you target an Ally that has the Trusted Upgrade on it, you lose the Upgrade and it gets discarded when the Ally goes back to your hand. When you take into consideration that both cards cost 1 resource, I personally normally just run Calling in Favors instead, since it returns the Ally card to your hand (effectively healing it) and also lets you search for your other useful allies. Trusted is nice to have, but I don't think it's effective right now to run it in addition to Calling in Favors.