I did this a couple of months ago (and at the time had about 99,000 less subs than 100k, so that's definitely not the threshold). To echo the general sentiment, if you're motivated enough to be on YT talk you know everything they are going to go over. Also, my partner manager is suuuuper flaky.
However, I would say it's worth it as it's not that time consuming (a couple 30 minute calls and some google hangout sessions with other creators if you want to participate in those) and then you have a dedicated contact/partner manager at YT who knows you and you are able to reach by phone or email with any questions or issues. I looked at it as expanding my professional network.
OH! Also, for those who were wondering, I asked how they chose channels to participate and was told that they have an algorithm that looks at sudden growth, sustained growth, and getting picked up in suggested videos.