For me thus far (granted, limited roller experience of 6 weeks of intermittent use), roller riding has not helped one bit on downhill fast mountain curves. I've felt much more comfortable on the rollers since I started for sure, from feeling really, really unstable (but still able to ride at a good clip), to riding confidently even at a good intensity, but for me, it translates zilch to fast mountain curves. Unless you really had egregious bike handling to begin with, the only way to ride those curves really fast is to practice riding them, faster and faster. The choice of line, degree of lean, and degree of braking needed on those sharp fast downhills is does not seem to be acquired by roller riding (which seems to improve pedal smoothness and balancing while pedaling.)

In terms of resistance, I don't deny that you can get crazy high resistances and all-out efforts on the rollers. Of course you can. But I don't think you're quite going all out vs a trainer, when you can put in that extra few % even when your form is deteriorating to mush, the world is blacking out, and everything screams QUIT! I'd have fallen off the rollers well before that point, and I would not even consider pushing that hard on the road as well due to safety concerns (ok on a totally closed course I would.) But on the trainer, where all is safe and secured, I can push into that zone.