For one thing, full membership requires you to tithe a fixed percentage (2%) of your income to the organization. Maybe I'm just being cheap, but I don't make a ton of money in research and that coupled with living in the most expensive area of the US makes me less inclined to seek out membership in their congregation.
I also feel weird about how they discuss their founder, Felix Adler. The number of times they invoke him seems almost like it's a cult of personality for him / he serves as a Jesus stand-in, and while I can't say I feverishly dislike the man, that was a huge turn off. If the group's observances were less centered around him and more concentrated on a broader variety of humanist figures, I would be able to get behind them a bit more.
The last issue I have with Ethical Culture is that it felt very monocultural / like an echo chamber. I'm fairly liberal by most standards, but I like to have exposure to individuals with different viewpoints from my own, be they conservative or libertarian. In Ethical Culture, any group other than die-hard liberals would meet with stiff opposition rather quickly and this was something I wasn't very comfortable with when I did attend services there. When I was religious, my church congregation was a much better mix of political perspectives (and even theological disagreements occurred occasionally).
In NYC, I've had much better experiences with the Secular Humanist Society of NY. It's much less formal, much more cost-effective for an expensive city (their meetings are in the back rooms of bars for the most part and yearly membership is only around $20 last time I checked) and it feels a lot more egalitarian / less hierarchical.