I guess what I am asking is, how much can you confide in a recruiter?
You need to remember / realize that a relationship with a recruiter is primarily a business relationship.
By that I mean that your recruiter's loyalty is to their career, their employer, and the client. Not necessarily in that order, but at least one (probably two) of those loyalties come ahead of their loyalty to you. They act like a friend would, but they are in sales and that's what good salespeople do.
Note that I'm not trying to belittle them, or you.
For it to be more than business, you have to work for them a while (years, not months).
What do you say, what don't you say, when they are checking in.
They are calling to chat with you either because it is time to do so or because you screwed up.
And you have to screw up bad for that to happen (you will likely already know what you did wrong if this is the case).
Unless something is really wrong, you should tell them everything is fine.
That is what they are expecting to hear. That is what they want to hear.
The work environment is sort-of the recruiter's business.
If you are having trouble with the people you work with, you can mention it.
I do not mean minor stuff like they go out to lunch and never invite you, I mean
If people aren't friendly or treat you as a second class citizen because you're a temp/contractor, you can mention that to the recruiter. It is good for them to know.
If you got a bad review from a superior and you can explain the reasons why that person was wrong, consider mentioning this. (They'll likely hear about it anyway.)
Otherwise, "Everything is fine, thanks!"
They are expecting to hear that everything is fine. They don't want to hear a bunch of minor stuff.
If you can't do the job because it is hard for you... don't tell. Suck it up and do the job they're expecting you to do. It is supposed to be hard, that's why they're paying you.
On the other hand... if you've missed deadlines (and other people aren't missing their deadlines) it would be time to say that you aren't happy... and are there other jobs?