Union season-ticket holder here.
Prokop is a monster in transition, can eat minutes, is a real shooting threat and also a capable passer. He moves pretty well, but is not an explosive skater. Great vision while skating, though, and able to find lanes and move through the neutral zone with ease. One of the best offensive defenseman I've seen in the ECAC in the last 5 years.
I would say he's an average defender in his own zone. He transitioned from F to D in the USHL, so maybe his game is still a work in progress there, but he is almost 24. He's not physical, can struggle with physical play, and has the occasional ugly giveaway, which makes sense considering how often the puck is on his stick.
I think his ceiling, considering his age, is a bottom-pair PMD/PP specialist. Definitely worth a contract to see what he can do at the pro-level.