Rossi Vipers, Jilly, remember them well - last ski my son ever rented (at Tahoe years ago, he was dark side exclusively by then). He rocked ‘em, too, like he’d never been off skis : ) Good ski.
However, I did have risers on K2 Fours, with Markers, and the weight was EXCRUCIATING. And I don’t think I EVER came close to boot-out anyway, probably wouldn't have w/o riser. This is more of a narrow-waist race ski thing now, where it once extended to recreational as well.
On the subject of “what if you’re on expert ski...” etc: Well, who’s defining expert here?
Basically, IMO, it boils down to these ski traits:
1. Ski is inherently stiffer. This might be a problem for anyone lighter in weight or without the skills to bend them.
2. Ski performs better at speed. Which might translate to them being a bit more sluggish at lower speeds.
3. Smaller “sweet spot.” Easier to get pitched into the back seat.
5. Livelier (but not always). Rebound sometimes is unexpected, as the skier doesn’t have the pressure control skills yet.
What I’ve seen: Even if good, advancing intermediates, if the ski is too stiff and prefers higher speeds, intermediates will tend to get thrown about a bit (if not more) in irregular terrain or bumps. If the ski wants to go faster, it might invite the skier to really crank it up, but without the proper skills and technique, which could invite some bad habits. I’ve seen skis get ahead of skiers, leading to back-seat emergency recovery tactics (heck, it happens to us all). But when or if this is happening - any of this - on a regular basis, it’s time to re-evaluate the appropriateness of the ski choice.
ETA: Deannat, I've seen skis listed side by side on merchant websites as system and flat (K2 for one), there is and always has been a price difference.