I got a game of Nevsky at Triangle Wargame Day a few months back, and it was a fascinating system that really forces you to balance what you're doing and where. The idea of scheduling things based on the availability of your vassals was easy enough to understand, and it does add a wrinkle to your planning. But it was predictable and could be planned for.
To me, the bigger issue was the unpredictability of what cards were about to come out each turn and how it would impact any battles being fought on that turn, because control of the road/river networks was so vital.
For instance, I was able to use a pair of battle cards (crossbows, and a hill to defend on) to quickly flip some likely victories by my opponent into pretty staggering defeats that closed up some access points he was counting on to get a further jump across my border.
I definitely want to play a longer scenario on this one to see more about how vassal availability works over a longer game.