Underwood szczur Rondon
Underwood: Totally unproven, but has such a high ceiling. Not many guys left at this stage with star possibility, and without having any strong evidence that they can't/won't ever get there.
Szczur: Way opposite extreme, very obvious limitations to his game and with no star possibility. But has a chance to become a good-range defender and a solid-OBP high-steal table-setter.
In my view, Underwood and Szczur are also opposites in that Szczur could fall off the map fast; if he can match his Daytona type profile at Tennessee, he'll stay solid, but if he hits as poorly in AA this spring as he did last August, even guys like me who still think he's got a chance will be dropping him fast. It wouldn't surprise me if Underwood could jump strongly, and a year from now he'd become more widely recognized as a pretty serious talent, even if he's still very distant.
Rondon:

If he does well, I want to be the poster who can claim I called it! And if he stinks, who will remember other than me?

But, he's going to be in the major leagues, and they think he has both enough velocity and enough control where they'll be able to use him some, not just waste another year. The odd confluence of circumstances for his draft selection enabled the Cubs to select a guy who has more tools and higher upside than is normally available in Rule 5 pick, while at the same time probably being more ready-now to actually be able to pitch in games.
If I was checking 4, Candelerio is the other one I wanted to check. Under the Underwood logic, he's got a chance to become a very strong player. Really good all-around hitters with power are hard to find, and he's got a chance. Even if he's limited at 3B and may end up in LF, RF, 1B, or C. Not sure if he's got the quickness, but he's got the arm to be an excellent catcher,and might be a really good hitter relative to catchers.