Hi there!
First off, let me say that I have absolutely adored the book, its rules and the factions within, and it has made it to my table as a staple people pull from to run. On that note, we recently decided to finally get together and actually organize a group for a conquest game and I have been elected as the Crypt Keeper. As I've been going through and answering questions, one that came up was on actions to take in the seasons, specifically "Ambush seems really strong and with no real downside. Why wouldn't I just always choose to ambush over normal siege?"
After reading the section a couple of times myself, I realized I also came to the same conclusion, that there seems to be no limit on the amount of ambushes you can make, and it just seems overall a better siege option you should always take. Is this a typo/oversight? Is there something I'm missing? If not I can always posit a vote to the group on how to deal with it, possibly make it a "once per season" action, but I wanted to see if I could get an official ruling first.