Renny is more forgiving now than it has ever been. I play there fairly regularly - it is good to go.
It will eat discs, but no more or less than anywhere else. There's really only one spot you can't go very deep into due to the swampiness. That's the downhill dogleg left on #9 (talking Renny Grey here). If you're not careful with disc selection, you could flip up and head straight out into that mess, and it's basically a lost cause in there. But that also depends on recent rain. It's been seriously wet for weeks, and we're drying out now, but...it can be sketchy down there.
All the stuff that used to be really tall and nasty (all along the sides and bottom of the famous ravine is cut short nowadays. It's possible to lose a disc in there, but again, it's not as bad as it used to be.
As for muddy places, I can't think of any that are especially bad. Maybe stay away from Dry Creek...?
Can't stress it enough: don's skip Renny.